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A cistern is filled by Pipe A and Pipe B together in 2.4 hours. Pipe A alone can fill the cistern at the rate of 100 litres per hour. Pipe B alone can fill the cistern in 4 hours. What is the capacity of the cistern?
500 litres
600 litres
1200 litres
1000 litres
600 litres
Calculate Pipe A's share of total capacity per hour relative to Pipe B, or use the relationship: Total Capacity = (Rate of A + Rate of B) ├Ч TimecombinedтАЛ, where RateBтАЛ = Total Capacity / 4.
Time taken by Pipe A and B together = 2.4 hours. Rate of Pipe A = 100 litres per hour. Time taken by Pipe B alone = 4 hours.
A1тАЛ+B1тАЛ=TcombinedтАЛ1тАЛ
Calculate Pipe A's share of total capacity per hour relative to Pipe B, or use the relationship: Total Capacity = (Rate of A + Rate of B) ├Ч TimecombinedтАЛ, where RateBтАЛ = Total Capacity / 4.
Confusing the rate of Pipe A (100 L/hr) with the total capacity, or forgetting to convert the combined time decimal correctly.
Define Rates
Let the total capacity be C. The rate of pipe A is given as 100 L/hr. The rate of pipe B is 4CтАЛ L/hr.
RateAтАЛ=100,RateBтАЛ=4CтАЛ
Combined Rate Equation
The cistern is filled together in 2.4 hours. Therefore, the combined rate is the total capacity divided by the time.
RateAтАЛ+RateBтАЛ=2.4CтАЛ
Solve for C
Substitute the rates into the equation: 100+4CтАЛ=2.4CтАЛ. Rearranging gives 100=C(2.41тАЛтИТ41тАЛ). Calculating the difference: 2.41тАЛтИТ41тАЛ=9.64тИТ2.4тАЛ=9.61.6тАЛ=61тАЛ.
100=6CтАЛтЯ╣C=600
B is correct because the calculated capacity of the cistern is 600 litres.
This concept of 'Rate = Work/Time' is identical to solving speed-distance-time problems where Rate is speed and Work is total distance.