The ratio of red marbles to blue marbles in a jar is 3 to 8. there are 40 blue marbles in the jar.

how many red marbles are in the jar?

a.5

b.15

c.24

d.35

_red_ = _3_              (set up problem) 
blue        8 

blue=40  R=red          (define terms) 

_ R_ = _3_                    (cross multiply) 
  40        8

8R=120            (divide by 8 on both sides)

R=15 

The answer is B.15