At the beginning of a year in a school the ratio of the number of boys and girls was 5 : 4. At the end of the year the ratio was 7 : 8 as 20 of boys left from the school and 20 girls newly joined. Find the strength of the school.

At the beginning of a year in a school the ratio of the number of boys of girls is 5 : 4

$\Rightarrow 5x:4x$

At the end of the year ratio was 7 : 8 as 20 of boys left from the school and 20 girls newly joined

5x – 20 : 4x + 20 = 7 : 8

40x – 160 = 28x + 140

40x – 28x = 140 + 160

12x = 300

x = 25

Boys $=5x=5\times 25=125$

Girls  $=4x=4\times 25=100$ 

Total students in school = 225