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
