In a family, the father took 1/5 of the cake and he had 4 times as much as others had, then the family members a. 16 b. 17 c. 18 d. none of these

There is one cake
It is divided among x family members, so each gets:
1/x
The father took 1/5 which is 4 times greater than each one's piece. So:
1/5 = 4(1/x)
1/5 = 4/x
x = 20
There are 20 family members so the answer is none of these