One way in which to approach this problem would be to treat it as an equation of ratios and to cross multiply:
[(4x + 15) / 5x)] = 1/2 could be written as:
4x + 15 1
----------- = ---
5x 2
Then 8x + 30 = 5x
3x = -30, and so x = -10 (answer). Be certain to check this answer through substitution!