The sum of 4 term of an G. P. Is 15/32 and sum of infinite term of same G.P. Is equal to 1/3
a(1-r^4)/(1-r) = 15/32
a/(1-r) = 1/3
divide the 1st by the 2nd
1-r^4 = 45/32
r^4 = 1-45/32 = -13/32
no real solution for r, since we can't take an even root of a negative
check your typing.