It costs $100 to join a fitness center plus a monthly fee. You spent $700 last year at the fitness center. How much was the monthly fee? Write and solve an algebraic equation to determine the monthly fee.

Let's call the monthly fee "m".

We know that it costs $100 to join the fitness center, so the total cost for the year would be:

$100 + 12m

We also know that you spent $700 last year at the fitness center, so we can set up an equation:

$100 + 12m = $700

Now we can solve for m:

$100 + 12m = $700

Subtract $100 from both sides:

12m = $600

Divide both sides by 12:

m = $50

Therefore, the monthly fee for the fitness center is $50.