Multiple choice

How long will two pipes together take to fill a cistern which they can separately fill in $20$ and $25$ minutes?

  1. $11\displaystyle\frac { 1 }{ 9 }$ min
  2. $10\displaystyle\frac { 1 }{ 9 }$ min
  3. $11\displaystyle\frac { 6 }{ 9 }$ min
  4. $11\displaystyle\frac { 2 }{ 9 }$ min
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The combined rate of the two pipes is 1/20 + 1/25 = 5/100 + 4/100 = 9/100 tanks per minute. The time taken to fill the cistern is the reciprocal of the rate: 100/9 minutes, which is 11 and 1/9 minutes.

AI explanation

Using the LCM method, let the total capacity be 100 units. The first pipe fills at 5 units per minute and the second pipe fills at 4 units per minute, giving a combined rate of 9 units per minute. Dividing the total capacity of 100 units by the combined rate of 9 units per minute gives 100/9 minutes, which equals 11 and 1/9 minutes.

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