The ratio of the number of boys to the number of girls in the school is 11 : 13 there are 72 less boys in the school how many kids are in the whole school

Lets:

x=number of boys.

y=number of girls

We can suggest this system of equations:

x/y=11/13 ⇒x=11y / 13

x=y-72

We can solve this system by substitution method

(11y/13) = y-72

lower common multiple=13

11y=13y-936

11y-13y=-936

-2y=-936

y=-936/-2

y=468

x=11y/13=11 (468) / 13=396

There are 396 boys and 468 girls in this school,

number of kids=number of boys + number of girls.

number of kids=396+468=864

Answer: there are 864 kids in this school.