How many hours will it take 3 pumps to empty a tank containing 60,000 litres of water?

Pumps	Litres	Hours
7	75,000	4
3	60,000	$x$

As the number of pumps has decreased, we will require more time. Therefore, from the numbers in the first column, we create the fraction that is greater than 1 (i.e. $\frac{7}{3}$ )

As the number of litres has decreased, we will require less time. Therefore, from the numbers in the second column, we create the fraction that is less than 1 (i.e. $\frac{60, 000}{75, 000}$ )

\begin{aligned} S o x & = \frac{7}{3} \times \frac{60, 000}{75, 000} \times 4 \\ = \frac{7}{3} \times \frac{4}{5} \times 4 \\ = \frac{112}{15} \\ = 7.4 \dot{6} h o u r s \end{aligned}

How many pumps will be needed to empty a tank of 90,000 litres in 3 hours?

Pumps	Litres	Hours
7	75,000	4
$x$	90,000	3

As the number of litres has increased, we will require more pumps. Therefore, from the numbers in the second column, we create the fraction that is greater than 1 (i.e. $\frac{90, 000}{75, 000}$ )

As the number of hours has decreased, we will require more pumps. Therefore, from the numbers in the third column, we create the fraction that is greater than 1 (i.e. $\frac{4}{3}$ )

\begin{aligned} S o x & = \frac{90, 000}{75, 000} \times \frac{4}{3} \times 7 \\ = \frac{6}{5} \times \frac{4}{3} \times 7 \\ = \frac{56}{5} \\ = 11 \frac{1}{5} p u m p s \end{aligned}

But as a part-pump is nonsense, we need 12 pumps