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HTML 

              
                <!DOCTYPE html>
<html>
	<head>
		<title>CALCUMAX</title>
		<link rel="stylesheet" type="text/css" href="css/index.css">
		<script type="text/javascript" src="./calculator.js" async="async"></script>
	</head>
	<body>
		<!-- The body of the calculator -->
		<div class="calc">
			<h2 style="font-family: Verdana, serif; font-weight: bolder;">CALCUMAX</h2>
			
			<div id="screen">
				<!-- This is where the input and output shows -->
				<h2 id="output"></h2>

			</div>

			<button value="CLEAR" id="clear" onclick="clearClicked()">CLEAR</button>

<!-- 		This is where the operands are: +,*, etc -->
			<div id="operands">
				<button class="opera" value="%" onclick="isclicked('%')">%</button> <br>
				<button class="opera" value="*" onclick="isclicked('*')">*</button> <br>
				<button class="opera" value="/"  onclick="isclicked('/')">/</button> <br>
				<button class="opera" value="-"  onclick="isclicked('-')">-</button> <br>

			</div>
			<!-- This is where the figures are located	 -->
			<div id="numbers">
				<button class="figures" value="7"  onclick="isclicked('7')">7 </button>
				<button class="figures" value="8"  onclick="isclicked('8')">8</button>
				<button class="figures" value="9"  onclick="isclicked('9')">9</button> <br>
				<button class="figures" value="4"  onclick="isclicked('4')">4</button>
				<button class="figures" value="5"  onclick="isclicked('5')">5</button>
				<button class="figures" value="6"  onclick="isclicked('6')">6</button> <br>
				<button class="figures" value="1"  onclick="isclicked('1')">1</button>
				<button class="figures" value="2"  onclick="isclicked('2')">2</button>
				<button class="figures" value="3"  onclick="isclicked('3')">3</button> <br>
				<button class="figures" value="0"  onclick="isclicked('0')">0</button>
				<button class="figures" value="."  onclick="isclicked('.')"><strong>.</strong></button>
				<span><button value="+" onclick="isclicked('+')">+</button> </span>
			</div>	
			<section>
				<button id="equalto" value="=" onclick="equalClicked()">=</button> 	
			</section>
		</div>
	</body>
</html>
!

CSS 

              
                * {
	box-sizing: border-box;
}
/*This styles the BODY of the CALCUMAX*/
.calc {
	width: 500px;
	height: 600px;
	background-color: cadetblue;
	margin: 50px auto;
	border-radius: 50px;
	padding: 7px;
	text-align: center;
}

/*This styles the DISPLAY SCREEN*/
.calc > #screen {
	background-color: white;
	width: 70%;
	height: 70px;
	margin: 20px auto;
	border-radius: 50px 15px;
	font-size: 18pt;
	text-align: right;
	word-wrap: break-word;
	padding-right: 10px;
}


/*This styles the CLEAR button*/
.calc > button {
	width: 100px;
	background-color: lightgreen;
	padding: 7px;
	margin-left:20px;
	cursor: pointer;
	border-radius: 5px;
	box-shadow: 0px 5px 10px 0px green, 0 6px 20px 0 rgba(0,0,0,0.19);
}
.calc > button:hover {
	background-color: orange;
	outline-color: black;
	width: 150px;
	color: white;
}

/*This styles the OPERANDS / OPERATORS row*/
.calc > #operands {
	display: block;
	padding: 30px 10px;
	width: 80px;
	height: auto;
	float: left;
}
.calc > #operands .opera {
	padding: 7px;
	margin-top:  15px;
	width: 75px;
	height: 50px;
	max-height: 50px;
	background-color: aqua;
	cursor: pointer;
	border-radius: 10px;
	
}
.calc > #operands .opera:hover {
	background-color: orangered;
	outline-color: black;
	width: 85px;
	color: white;
	border-radius: 20px 10px;
}

/*This styles the FIGURES div*/
.calc > #numbers {
	padding-top: 20px;
	width: 350px;
	float: right;
}
.calc > #numbers .figures {
	padding: 10px;
	margin-top:  15px;
	width: 75px;
	height: 40px;
	background-color: khaki;
	cursor: pointer;
	border-radius: 10px;
	margin: 10px;
	
}
.calc > #numbers span button {
	height: 70px;
	width: 50px;
  	background-color: darkkhaki;
	border-radius: 20px;
	cursor: pointer;
}
.calc > #numbers .figures:hover {
	width: 100px;
	background-color: aqua;
	border-radius: 20px 5px;
}
.calc > #numbers span button:hover { 
	background-color: gainsboro;
	border-radius: 20px;
	box-shadow: 0px 2px 12px 0px green;
}

/*
This styles the EQUAL TO section*/
.calc > section button {
	background-color: rosybrown;
	width: 200px;
	padding: 7px;
	border-radius: 10px;
	cursor: pointer;
}
.calc > section {
	clear: both;
	width: 100%;
}
.calc > section button:hover {
	width: 250px;
	border-radius: 20px 5px;
	background-color: firebrick;
}
@media screen and (max-width: 520px) and (min-width: 320px) { 
	.calc {
		width: 100%;
		padding: 5px;
	}
	.calc > #operands {
	display: block;
	padding: 30px 10px;
	width: 80px;
	height: auto;
	float: left;
}
	.calc > #operands .opera {
	padding: 7px;
	margin-top:  5px;
	width: 45px;
	height: 50px;
	max-height: 50px;
	background-color: aqua;
	cursor: pointer;
	border-radius: 10px;
	}
	.calc > #numbers .figures {
	padding: 7px;
	margin-top:  10px;
	width: 50px;
	height: 40px;
	background-color: khaki;
	cursor: pointer;
	border-radius: 10px;
	margin: 10px;
	}
	.calc > #numbers {
	padding-top: 20px;
	width: 250px;
	float: right;
	}
	.calc > #operands .opera:hover, .calc > #numbers .figures:hover {
		width: 50px;
	}
	.calc > button:hover {
		width: 100px;
	}

}
!

JS 

              
                let display = document.getElementById('screen');
function isclicked(value) {
        if(display.innerText === 0) {
            display.innerText = value;
        } else {
            display.innerText += value;
        }
        return display.innerText;
}
let equals = document.getElementById('equalto');
const equalClicked = () =>{
    if(display.innerText !== "") {
        display.innerText = eval(display.innerText)
    } else {
        display.innerText = "";
    }
}
const clearClicked =() => {
    display.innerText = "";
}
!
999px

Console