Answer
You have up to \$130 left to spend each month.
Work Step by Step
The amount of money that you spend and save should be less than or equal to the amount of money that you earn.
The amount of money that you spend/save each month is the sum of the club fee, your monthly savings, and your miscellaneous spending. Let m be the amount you have left to spend on miscellaneous things.
$\ \underbrace{20+100+m}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \underbrace{\leq}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \underbrace{250}$
total spend/save $\ \ \ $ should not exceed $\ \ \ $ earnings
Solve the equation.
$20+100+m\leq250\longrightarrow$ combine like terms
$120+m\leq250\longrightarrow$ subtract 120 from each side
$120+m-120\leq250-120\longrightarrow$ subtract
$m\leq130$
