Since you want both sides to equal each other, you can set up the following equation:
chess club members = computer club members
500 - 30x = 150 + 20x
x represents the amount of years that have passed. The coefficients for x represent the loss or gain of members per year.
Add 30x to both sides.
500 = 150 + 50x
Subtract 150 from both sides.
350 = 50x
Divide both sides by 50 to solve for x.
x = 7
It will take 7 years for both clubs to have the same amount of members.
