Question

Wilda can do a job in 4 hours that karla needs 5 hours to do. If wilda works 1 hour before karla joins her, how long will it take the two of them to finish the job?

Answer 1

Wilda can do the job in 4 hours ... she does 1/4 (or 0.25) of it each hour.
Karla can do it in 5 hours ... she does 1/5 (or 0.2) of it each hour.

Wilda worked alone for 1 hour ... 0.25 of the job was done before her helper
arrived.  So only 0.75 of the job remained to be done together.

Working together, they accomplish (0.25 + 0.2) = 0.45 of the job in 1 hour.

How many times do they need to do  0.45  of the job in order to finish
the  0.75  of it that remains ?  That's the number of hours it will take them,
working together.

0.75 / 0.45  =  1 and 2/3

After Wilda worked alone for 1 hour and Karla came along to join her, it will
take them another  1 hour and 40 minutes to finish the job and go for a swim.