Example 7 6 pipes are required to fill a tank in 1 hour 20 minutes. How long will it take if only 5 pipes of the same type are used?
Let’s first convert time into minutes
1 hour 20 minutes = 1 hour + 20 minutes
= 60 minutes + 20 minutes
= 80 minutes
It is given that 6 pipes fill tank in 80 minutes
We need to find time taken if 5 pipes are used
Let time taken be y minutes
Thus, our table looks like
As we increase the number of pipes,
the time taken to fill the tank decreases
∴ Number of pipes and time taken are in inverse proportion
6 × 80 = 5 × 𝑦
(6 × 80)/5 = 𝑦
6 × 16 = 𝑦
96 = 𝑦
𝑦 = 96 minutes
Converting to hours
(𝑥1𝑦1=𝑥2𝑦2)
𝑦 = 96/60 hours
𝑦 = 1 hours 36 min
∴ 5 pipes will fill the tank in 96 minutes (1 hour 36 minutes)

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 13 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.

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