Question 131553
The number of boys: b
The number of girls: g
There are 15 more girls than twice the number of boys:  {{{g = 2b + 15}}}
Presuming that all 561 students are classified as either boys or girls (and I would say this seems like a reasonable presumption): {{{g + b = 561}}}


Take the expression developed for the number of girls in terms of the number of boys and substitute it:
{{{cartoon(red(g)+b=561,red((2b+15))+b=561)}}}


Then solve for b:
{{{(2b+15)+b=561}}}


{{{3b+15=561}}}


{{{3b=546}}}


{{{b=182}}}


So now we know that there are 182 boys.  We can just subtract from 561 to get 379 girls, or double the number of boys {{{182*2=364}}} and add 15 to get 379.  Doing it both ways checks the answer.