Let us assume total number of students were surveyed = x.
1/4 of x students preferred red = 1/4 x.
1/8 of of x students preferred blue = 1/8 x
Remaining students = (x - 1/4 x - 1/8 x)
3/5 of the remaining students chose green that is 3/5 of (x - 1/4 x - 1/8 x).
15 students preferred green.
So, we can setup an equation:


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Dividing both sides by 3.

Multiplying both sides by 8, we get
5x = 200.
Dividing both sides by 5, we get
x= 40.
Therefore, 40 students were surveyed.