Question 91870
Assume it takes them T hours working together.

then in one hour the first person can finish {{{1/8}}}
the second person can finish {{{1/12}}} in the same one hour

working together, the first person will spend T-2 hours, the second person will spend T hours

the portion finished by first person is {{{(1/8)*(T-2)}}}
the portion finished by second person is {{{(1/12)*T}}}

together they finish the whole thing represented by 100% which is 1

so we have  {{{(1/8)*(T-2) + (1/12)*T = 1}}}

multiply both sides by 24 we get

3(T-2) + 2T = 24

solve this equation we get T = 6 hrs. 

( I assume you know how to solve one variable equation)