Find the probability of getting exactly three tails when four coins are tossed.

explain!

75% or three fourths 

So we know that at least 3 heads means 3 or 4 heads 

(3 heads)=1/2*1/2*1/2*1/2*4C3=4*1/16=4/16 
(4 heads)=1/16 

4/16+1/16=5/16

So the answer would be 5/16