Let a = number of chocolate candies and b = number of sour candies. That gives us two equations
2a + 0.75b = 75 (75 is the total weight, since $1.50 * 50 = 75)
a + b = 50 (Need 50 candies in total)
a = 50 - b (Solve the second equation for a)
Now plug a = 50 - b into the first equation and solve for b:
2(50 - b) + 0.75b = 75
100 - 2b + 0.75b = 75
-1.25b = -25
b = 20
And with b = 20, solve for a:
a = 30
Therefore, we will need 30 lbs of chocolate candies and 20 lbs of sour candies