Let amount of water in lbs is x
so, mass of solution = mass of water + mass of sugar
= (x + 0.5) lbs
concentration of syrup = mass of sugar/mass of solution × 100
75 = 0.5/(x + 0.5) × 100
or, 75/100 = 0.5/(x + 0.5)
or, 3/4 = 0.5/(x + 0.5)
or, 3x + 3 × 0.5 = 4 × 0.5
or, 3x + 1.5 = 2
or, 3x = 0.5
or, x = 0.167 lbs
hence, amount of water is 0.167 lbs
Hope this helps :)