Question

Dave can complete a sales route by himself in 4 hours. James can do the same job in 5 hours. How long will it take them to do it working together?

Answer 1

We can solve this problem by calculating the individual rate of working and equate it to their total rate of working.

If Dave can complete a sales route in 4 hours, then his working rate is

$\frac{1}{4}$

Also, if James can do it in 5 hours, then his working rate is

$\frac{1}{5}$

Let
$x$
be the hours that both will use to complete the sales route,

Then rate at which both completes this task is
$\frac{1}{x}$


Meaning if we add their individual rates we should get

$\frac{1}{x}$

That is;

$\frac{1}{4} + \frac{1}{5} = \frac{1}{x}$

The LCM is
$20x$

So let us multiply through with the LCM.

$20x \times \frac{1}{4} + 20x \times \frac{1}{5} =20x \times \frac{1}{x}$

$5x + 4x = 20$

We simplify to get,

$9x = 20$

Dividing through by 9 gives;

$x = \frac{20}{9}$

$x = 2\frac{1}{9}$

Therefore the two will complete sales route in
$2 \frac{1}{9}$
hours.