This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
The answer is 16.. 

think of a number that when you multiply by 3 the answer is 15..15 / 3
the answer is 5 so there must be 5 boys left for the girls to be thrice more than girls.. the question is about how many boys should leave so tou'll just subtract 5 FROM 21
The answer is 16
you just need to divide 15 to 3 ....15/3
 ur gonna find out that the answer is 5
then u need to subtract 5 from 21
21-5 =16