1.

4 Pumps are required to fill a tank in 1 hr 30 min. How long will it take if only 3 pumps of the same type are used?

Answer»

Let the time be ‘x’. 1 hr 30min = 60 + 30 = 90 minutes , If the number of pumps are decrease, the time taken to fill the tank increases. So, the number of pumps and time taken to fill the tank are in inverse proportion.

Number of pipesTime taken to fill (min)
490
3x

By taking inverse proportion, 

4 : 3 = x : 90 

Thus, 

∴ 4 × 90 = 3 × x 

⇒ 3 × x = 4 × 90

⇒ x = 4 x 90/3 = 120 min

∴ 3 pumps will fill the tank in 120 min or 2 hrs. 

Observe this: 

In inverse proportion, product is always constant

∴ 4 × 90 = 3 × x 

⇒ 3 × x = 4 × 90

⇒ x = 4 x 90/3 = 120 min


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