The ratio of number of girls to the number of boys is 3:5
Sum of the terms of the ratio=3+5=8
The number of girls in the school== and,
The number of boys in the school==
Let the number of new girls admitted be x,then the number of girls become (270+x).
After admitting 18 new boys,the number of boys become 450+18 i.e., 468
According to the given,
Hence the number of new girls admitted is 42