As the title implies, we can set up a system of two equations for our two variables - the number of children (C) and adults (A) that entered the park. One equation will describe the number of people that entered the park, while the other will describe the amount of money collected:
C + A = 210
1.5C + 4A = 590
We can solve this several different ways. I will solve by rearranging the top equation for A, then substituting into the bottom equation.
A = 210 - C
1.5C + 4(210-C) = 590
1.5C + 840 - 4C = 590
-2.5C + 840 = 590
-2.5C = -250
C = -250/-2.5
C = 100 children
By substituting this value into our first equation, we can find the number of adults.
100 + A = 210
A = 110 adults
I hope this helped!