Asalesman's commission varies directly as his sales. if the commission is $10 for $100 in sales, find the commission for $150 in sales.

You are given a commission of $10 for $100 in sales. You are to find the commission for $150 in sales. It is stated in the problem that the salesman’s commission varies directly as his sales. Directly means proportional, and so this relationship has a constant. Let us say the salesman commission is c and his sales is s. Let us also denote k as our constant.  

C = ks
$10 = k($100)
k = 0.1  

We need to find the salesman commission in $150 sales.  

C = ks
C = (0.1)($150)
C = $15