4 months.
Making these into an mx + b = y equations, this is my work:
20x+50 = 30x+10
Subtract 20x from each side.
50 = 10x+10
Subtract 10 from each side.
40 = 10x
Divide each side by 4.
x = 4.
You can then input x into each equation and get 130$ after 4 months.
