#### Answer

The store must sell 40 bicycles each month to break even.

#### Work Step by Step

Let x = the number of bicycles sold and let y = the number of dollars of expense or income.
Expense: $y = 60x + 2400 $
Income: $y = 120x $
Use substitution since both equations are solved for $y$.
$60x + 2400=120x$
$60x=2400$
$x=40$
The store must sell 40 bicycles each month to break even.