Question

Manny can assemble a computer by himself in 50 minutes. Lucy does the same job in 45 minutes. How long will it take to assemble a computer working together

Answer 1

Manny can assemble a computer in 50 minutes. That means, in a minute Manny does $\frac{1}{50}$ job.

Lucy can assemble a computer in 45 minutes. That means, in a minute Lucy does $\frac{1}{45}$ job.

If they are working together, in a minute they will do:
$\dfrac{1}{50} + \dfrac{1}{45}$
$= \dfrac{9}{450} + \dfrac{10}{450} \text{(equalize the denominator)}$
$=\dfrac{19}{450}$
In a minute they will do $\frac{19}{450}$ job

To finish the job they will do it in $\frac{450}{19}$ minutes or about 24 minutes.