Answer: 15/16
Explanation:
There will be 16 outcomes for 4 coins=2^4
Probability of only head is : P(C)=1/16
P(C) + P(C') = 1
P(C') = 1 - P(C)
P(C') = 1 - 1/16
Take the l.c.m
P(C') = (16-1) / 16
P(C') = 15/16
So the probability of getting at least 1 tail is 15/16