My guideline is to see how many variables can I eliminate at the same time i.e. by a single addition or subtraction operation.
As for not touching an equation, it depends on what variables the eqn involves. In this case h = 7s wasn't necessary to eliminate w and s from the first equation.
In this case adding two simple equations and eliminating the large one makes sense.
My solution was not super quick, but took me less than a minute.. ballpark.
Hope this helps, but then again I am not an expert at Quant... so other experts will probably be more helpful.
haas thanks... makes sense but takes me a bit to do it... will practice this...
do you just start working randomly with equations, or your prefer the ones with only additions first... or the ones that have different variables... or does it even matter?
f.e h=7s and h=w+4... shouldn't be touched because each is solving for h... am I right?