Question

a school has 1260 student. the ratio of the number of girls to number of boys is 2:3.if 180 new student join school the ratio changes to 5:7. how many new boys joined the school.

Answer 1

Initially, there were $\frac{3}{5}*1260=3*252=$ 756 boys in school.

After the admission of 180 more students into the school, the number of boys increases to - $\frac{7}{12}*(1260+180)$

                      $= \frac{7}{12}*1440$

                      $=7*120$

                      $=$ 840

Difference = $840-756$
 
                   = 84 (ans.)