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
