Question

6 water pumps fill a water tank in 15 hours if they work together. a) If one of these pumps is not working, how long will the 5 water pumps take to fill the tank? b) If we want the tank filled in 9 hours, how many water pumps do we need?​

Answer 1

a) 18 hours

b) 10 water pumps

Explanation:

• We notice that we are in an inverse proportion situation

because when the number of pumps increase the number

of hours taken to fill the tank decrease and vice versa.

Formula :

in an inverse proportional relation :

the product of the two variables = a constant (k)

……………………………………………………………………………

let p be the number of pumps

and h be the number of hours

By applying the formula we get :

p × h = k

Then the constant of proportionality :

k = 6 × 15 = 90

a)

5 × h = 90

Then

h = 90/5

= 18

b)

p × 9 = 90

Then

p = 90/9

= 10