Okay, I guess it's the second part which bothers you.
If you want, I'll go into the process of deriving it.
So... Alex takes 'a' hours to complete the job alone.
Barry takes 'b' hours to complete the same job alone.
Going by the standard rate formula, we know that "Total work done = Rate of work done x Time taken".
(Similar to "Total distance travelled = Speed x Time")
Let's say that the total work to be done is '100'. The rate of completion of work of Alex would be: Rate = 100/a
To take actual numbers, say Alex takes 5 hours to complete the work '100'. He would be working at a rate of 100/5 = 20/hour. Makes sense?
Using the same logic, Barry would be having a rate of 100/b. Let's say he takes 2 hours. The rate would be 50/hour
Let's say they now work together. This means that they are adding their rates. In one hour, Alex would be doing '20' of the '100' and Barry would do another '50' of the '100' for a total of 70 in one hour! This means their rates were added up.
This brings it back to:
Rate of Alex + Rate of Barry = Rate work is being done
Substitute what we know:
100/a + 100/b = t
Where t is their combined rates. The work, however is still 100, and thus, the time, T, to complete the work together (using the rate formula) would be:
\(\displaystyle 100 = t \times T\)
t = 100/T
Thus we have:
100/a + 100/b = 100/T
We have a common numerator and it was a number we introduced anyway. This can be cancelled:
1/a + 1/b = 1/T
So, maybe the derivation is more complex (I find it more complex myself XD) and I think it's just that you should remember the final formula and understand the steps. Though if you don't understand it straight away, it doesn't matter, you still have time to do so
Bottom line,
a = b + 3
1/a + 1/b = 1/3.6
Do you know how to solve those two using simultaneous equations?