You can view full video solution here – https://youtu.be/8Db_xy8RFD4

Let u = number of kids from 13-16 years old

1-6 years old     [u][u][u][u][u][u]  = 6u     (1-6 yo twice as many of 7-12yo)

7-12years old    [u][u][u] = 3u    (7-12yo thrice as many of 13-16 yo)

13-16years old  [u] = u

6u(3) + 3u(6) +u(12) = 1440

18u +18u+12u = 1440

48u = 1440 => u = 30

6u = 6x 30 = 180

Hence money collected from 1-6 years old category is 180x 3 = $540