SOLUTION: A bowl contains red marbles and green marbles. Sue knows there are 150 red marbles. After mixing the marbles, she randomly selects 50 marbles, 15 of which are red. Approximately ho

Assuming that the ratio of red to green marbles is the same in the sample selected (50) as in the bowl.

In the sample, 15 out of 50 are red. i.e. fraction of red marbles = 
Totally there are 150 red marbles. If this is also 3/10 of the total marbles in the bowl, and assuming total marbles are x, we get an equation




So there are approximately  marbles in the bowl.

Hope this helps.