After 16 downloads the charges will equal each other.
First set up an equation
x = Downloads
12 + 1x = 5 + 1.5x
Solve
12 + 1x = 5 + 1.5x <- Rewrite your equation
-5 Â Â Â Â Â -5 Â Â Â Â Â Â <- Subtract 5 from both sides
 8 + 1x = 1.5x    <- Simplify
   -1x   -1x     <- Subtract 1 from both sides
    8 = 0.5x    <- Simplify
    8 = 1/2x    <- Convert 0.5 to a fraction to make the next step easier
    16 = x       <- Multiply both sides by 2 to divide by
                 a fraction