A sum of money at simple interest amounts to $ 645 in 3 years and to $690 in 4 years. The sum is:
Select:
A) $153
B) $510
C) $598
D) $549
E) $675
B   $510   S.I. for 1 year = $(690 - 645)= 
<gadget id="calculator">(690 - 645)</gadget>
<output>45</output>
$45.   S.I. for 3 years= $(45 x 3) = $135.   Principal = $(645 - 135)= 
<gadget id="calculator">(645 - 135)</gadget>
<output>510</output>
$510.
<result>B</result>
A sum of money at simple interest amounts to $ 645 in 3 years and to $690 in 4 years. The sum is: