if the ratio of red marbles to blue marbles in a jar is 3 to 8, the meaning is for every 3 red marbles there are 8 blue marbles.
3 red marbles------------------8 blue marbles.
ratio=3/8
we have 40 blue marbles; we have to compute the number of red marbles.
1) Method 1; by the rule three.
3 red marbles-----------------8 blue marbles
x----------------------------------40 blue marbles
x=(3 red marbles * 40 blue marbles) / 8 blue marbles=15 red marbles.
answer: B. 15
Method 2: the ratio of red marbles to blue marbles is
ratio=number of red marbles / number of blue marbles
ratio=3/8
if we want to compute the number of red marbles we have to multiply the number of blue marbles by this ratio.
number of red marbles=ratio (red/blue)* number of blue marbles
number of red marbles=(3/8)*40=15
Answer: B.15