Please solve the sum.

for this A.P.,

 a=first erm=7 yrs

d=common diff=3 months=1/4 yrs

let there be n students

So sum of their ages

=n/2[2a+(n-1)d]

=n/2[14+{(n-1)/4}]

=250(given)

so solving we get,

(n+80)(n-25)=0

so

n=25