Problem: a cistern has a leak which would empty the cistern in 20 minutes . a tap is turned on which admits 7 liters a minute into the cistern , and it is emptied in 24 minutes . how many liters does the cistern hold ?
Rationale: "1 / x - 1 / 20 = - 1 / 24 x = 120 120 * 7 = 840 answer : d"
annotated_formula: multiply(24, 20)