Every time I come across a GMAT combined work problem it throws me off. I found a strategy that I thought would work for me, but I came across a few problems in which I did not know how to apply the strategy. Can someone please help me out?

The strategy uses a table in which the formula Velocity = Distance/Time is used.

Since the independent people or things are doing the same activity, the distance remains a constant, which is 1.

In a simple problem, this is how the strategy would be applied:

If Sam can finish a job in 3 hrs and Mark in 12 hrs, in how many hrs can they finish the job if they worked on it together at their respective rates?

Solution:S 1/3 1 3

V D T

M 1/12 1 12

S+M V(s+m) 1 X

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S 1/3 1 3

M 1/12 1 12

S+M 5/12 1 T

So, to solve for V(s+m) 1/3 +1/12 = V(s+m)

5/12 = V(s+m)

Now, to solve for the time V= D/T

5=1

12 T Solving for T, T = 2 2/5 hrs

I would like to use a consistent approach for solving these types of problems so that I am not confused at the time of taking the GMAT. Thanks for your help.

So, using this methodology, how would I solve the following problems?

Working together, Bill & Tom painted a fence in 8 hrs. Last year Tom painted the fence by himself. The year before, Bill painted the fence by himself, but it took 12 hrs less than Tom took. How long did Bill and Tom take when each was painting alone?

Last Thursday John assembled chairs at a rate of 3 chairs per hour for part of the day and Larry assembled no chairs. Last Friday, Larry assembled chairs at a rate of 4 chairs per hour for part of the day and John assembled no chairs. If during these two days, John and Larry assembled a total of 25 chairs over 7 hours, how many chairs did John assemble last Thursday?