Question

two pipes separately can fill an empty cistern with water in 20 minutes 30 minutes respectively. what part of the cistern can be filled up by the two pipes together in 1 minute?

Answer 1

Answer:

Given:

Two pipes can fill a cistern in 20 minutes and 30 minutes.

Calculation:

Let the capacity of cistern be ‘100’.

% Cistern is filled by 1st pipe in one minute = 100/20

⇒ 5%

% Cistern is filled by 2nd pipe in one minute = 100/30

⇒ 3.33%

% Cistern filled by 1st and 2nd pipes in one minute

⇒ 5% + 3.33%

⇒ 8.33%

Cistern is totally filled by 2nd pipe in last 10 minute.

⇒ 3.33% × 10

⇒ 33.3%

Remaining filled by 1st pipe and 2nd pipe

⇒ 100 – 33.3

⇒ 66.6%

Time taken to fill remaining by both pipe

⇒ 66.6/8.33

⇒ 8 minutes

∴ The first pipe must be turned off in 8 minutes.