To solve this problem we can use equation.
Lets say that x is number of adults
We know that childres was 3 times more, so 3x. Knowing price of ticket for children and adult we can create eq
7*x+3*3x=6000
Now we just have to simplify
7x+9x=6000
16x=6000 /:16 divide both side by 16
x=375
We got number of adults
Now we want to find number of children so
375*3=1125
Finally, total number of people
375+1125=1500 - its the result