Sara bought both German chocolate and Swiss chocolate for some cakes she was baking. The Swiss chocolate cost $3.5 per pound, and German chocolate cost $2.5 per pound. if the total the she spent on chocolate was $70 and both types of chocolate were purchased in whole number of pounds, how many total pounds of chocolate she purchased? Select.
A) 7
B) 8
C) 10
D) 24
E) 15

Rationale: if there were all the expensive ones , 3.5 . . . . there would be 70 / 3.5 or 20 of them but since 2.5 $ ones are also there , answer has to be > 20 . . . . if all were 2.5 $ ones , there will be 70 / 2.5 or 28 . . so only 24 is left ans d . .
Answer: D