story problem for pre algebra using fractions

Question

A roofer and an apprentice are roofing a newly constructed house. In one day, the roofer completes
3
5
of the job and the apprentice completes
1
4
of the job. How much of the job remains to be done?

Answer 1

If they had each done 1/2 of the job then we know 1/2 + 1/2 is 1 so the whole job would be done.

 

Although the problem is not as simple, we can solve it the same way, by adding the two fractions together. 

 

3/5 + 1/4

 

First we need to get a common denominator. The LCD for 4 and 5 would be 20

 

so I rewrite as 12/20 + 5/20 which equals 17/20.

 

This is how much the roofer and apprentice have completed, so how much would be left?

Answer 2

 3/5  + 1/4 =(12+5)/20 =17/20

 

1 -17/20 =3/20

 

3/20 part of job is remain to done