If 4 electricians can complete a job in 8 hours how long will it take 3 electricians to complete the same job

We know that

if 4 electricians can complete a job in 8 hours
then
100% of a job > 8 hours
x> 1 hour
x=100/8> x=12.5%

divide by 4 electricians
12.5%/4> 3.125%
that means each electrician can complete 3.125% of a job in 1 hour

therefore
3 electricians> 3*3.125%> 9.375% in 1 hour
so
9.375%> 1 hour
100%> x
x=100/9.375> x=10.67 hours

the answer is
10.67 hours

Where are the following equations?