algebra
posted by Anna64 .
Of 480 consumers polled, some purchase ice cream, some purchase frozen yogurt, and some purchase both. If 260 people polled purchase ice cream and 200 purchase both ice cream and frozen yogurt, how many people purchase frozen yogurt?

Assuming that of the 480, everybody had either ice cream or yogurt
number for ice cream  A
number for yogurt  B
N( A or B) = N(A) + N(B)  N(A and B)
480 = 260 + N(B)  200
N(B) = 420
confirm the answer is corrrect by making a Venn diagram. 
n+260200 = 480
n = 420
420 bought just yogurt