The total rental cost is $120.
Let x be the amount each person pays and y be the total cost. Our first equation would be
4x = y.
The second equation would be
6(x-10) = y.
Since these are set equal to the same variable, we can use substitution:
4x = 6(x-10)
Using the distributive property, we have
4x = 6*x - 6*10
4x = 6x-60
Subtract 4x from both sides:
0 = 2x-60
Add 60 to both sides;
60 = 2x
Divide both sides by 2:
30 = x
This means each person pays $30 (in the first equation). 4(30) = 120 for y.
With 4 people, each person pays $30; with 6 people, each person pays 30-10=20.