Let’s define our variables: x represents child passes and y represents adult passes.
Now, let’s make some equations with the provided information.
First, let’s deal with the money
12x+15y=4140
Next, the amount of passes
x+y=300
Then, take the second equation, and subtract y over.
x=300-y
Now, we can substitute the equation above in for x in the first equation
12x+15y=4140
12(300-y) +15y =4140
Distribute the 12
3600-12y+15y=4140
Combine like terms, then subtract 3600 over
3y=540
Divide by 3 on both sides
y=180
So, 180 $15 entry passes were sold