Problem: after a storm deposits 120 billion gallons of water into the city reservoir , the reservoir is 50 % full . if the original contents of the reservoir totaled 280 billion gallons , the reservoir was approximately what percentage full before the storm ?
Rationale: "when the storm deposited 115 billion gallons , volume of water in the reservoir = 280 + 120 = 400 billion gallons if this is only 80 % of the capacity of the reservoir , the total capacity of the reservoir = 400 / 0.5 = 800 billion gallons therefore percentage of reservoir that was full before the storm = ( 280 / 800 ) * 100 = 35 % option c"
annotated_formula: multiply(divide(280, divide(add(120, 280), divide(50, const_100))), const_100)