It takes an old copier 15 hours longer than the new onw to finish a job.It takes them 4 hours to finish a project together. How long will it take each of them to copy the job alone?(I resent this,I couldn't understand the last one)

Solution:

It takes X hours for the old one alone,

It takes (X-15) hours for the new one alone

We assume the work is 1

so Time taken to finish a project together is 1/X + 1/(x-15)

so 1/X + 1/(x-15)= 1/4

we calculate it

X=20

so

It takes 20 hours for the old one alone,

It takes 5 hours for the new one alone