Javascript getelementbyid multiple elements same id

[moeong]

This is actually my school project. I doing a table form and want to loop at Course Marks column and Course Grade Point column. Somehow i'm unable to to figure it out. Need a help to fix my work. Thank in advance.

<table border="1">
<tr>
	<th>Course Marks</th>
	<th>Course Grade Point</th>
	
</tr>
<tr>
	<td><input type="text" id="Mark1"  onfocusout="myFunction1()"></td>
	<td><input type="text" id="Point1" ></td>
	</tr>

<tr>
<td>
	<td><input type="text" id="Mark2" onfocusout="myFunction1()"></td>
	<td><input type="text" id="Point2"  ></td>
	>

Script

function myFunction1() {
var gradep=0.0;
var x = document.getElementById("Mark1");
var y = document.getElementById("Point1");
y.value = typeof x.value;
va= Number(y.value);
y.value = typeof va;
x= Number(x.value);
if (x>=80 && x<=100){
		gradep=4.00;
		}
else if (x>=75 && x<=79){
		gradep=3.67;
		}
else if (x>=70 && x<=74){
	gradep=3.33;
		}
else if (x>=65 && x<=69){
	gradep=3.00;
		}
else if (x>=60 && x<=64){
	gradep=2.67;
		}
else if (x>=55 && x<=59){
		gradep=2.33;
		}
else if (x>=50 && x<=54){
		gradep=2.00;
		}
else if (x>=45 && x<=49){
		gradep=1.67;
		}
else if (x>=40 && x<=44){
		gradep=1.33;
		}
else if (x>=35 && x<=39){
		gradep=1.00;
		}
else if (x>=0 && x<=34){
	gradep=0.00;
		}
y.value= gradep.toFixed(2);
		
		}