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

there are 864 kids in this school.