OK, so embarrassingly I've forgotten how to do this type of problem which I'll generalize:
$n$ workers of type $A$ can do a job in $x$ hours. $m$ workers of type $B$ can do the same job in $y$ hours. How long would it take $n_1$ workers of type $A$ and $m_1$ workers of type $B$ to do the job working together?
Logically I can see that $1$ type $A$ worker should spend $n$ times as much time as $n$ workers take. Same with $B$. So a type $A$ worker can do the job in $nx$ hours and a type $B$ worker could do the job in $my$ hours. I just don't seem to be able to figure out how long a mixture of type $A$ and type $B$ workers take.
Could someone please explain the process to me? Thanks.