x - volume when tank is 1/3 full
y - volume of entire tank
A) x/y = 1/3 (or 3x = y)
B) [(x + 500 liters) / y] = 3/4
Substituting A) into B)
B) [(x + 500 liters) / 3x] = 3/4
B) (x + 500 liters) = 9x / 4
4x + 2,000 = 9x
2,000 = 5x
x = 400
(400+500)/y = 3/4
3y = 1,600 + 2,000
y = 1,200
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Double Check
when tank is 1/3 full = x/y = 400 / 1,200 = 1/3
After adding 500 liters
900 / 1,200 = 3/4
