A tank is to be filled with water. When the tank is one-sixth full, 130 gallons of water are added, making the tank three-fifths full. How many gallons does the tank contain when it is completely full?
Let the full tank = T
Solve for T:
T/6 + 130 = (3 T)/5
Put each term in T/6 + 130 over the common denominator 6: T/6 + 130 = T/6 + 780/6:
T/6 + 780/6 = (3 T)/5
T/6 + 780/6 = (T + 780)/6:
(T + 780)/6 = (3 T)/5
Multiply both sides by 30:
(30 (T + 780))/6 = (30×3 T)/5
5 (T + 780) = (30×3 T)/5
5 (T + 780) = 6×3 T
5 (T + 780) = 18 T
Expand out terms of the left hand side:
5 T + 3900 = 18 T
-13 T = -3900
T = 300 Gallons when the tank is full.