Dave has $15 to spend on an $8 book and two birthday cards (c) for his friends. how much can he spend on each card if he buys the same card for each friend?

Funds = Expenses

Funds is $15, Expenses are the book and the two cards. We know how much the book costs ($8). We do not know how much each card costs, so we'll call that "x".

$15 = $8 + 2x

Subtract $8 from both sides of equation: $15 - $8 = $8 - $8 + 2x

Simplify: $7 = 2x

Divide both sides of the equation by 2: $7/2 = 2x/2

Simplify: $3.50 = x

So, each card costs $3.50