St2 of the above kind that ‘gives us relation between a part and the whole’ can be easily expressed into a relation between the two parts. IMO this skill is nicely rewarded on the gmat. For instance in the above example St2 says day shift pay is 50% of the total, meaning –

Day shift pay = night shift pay

d*w = (x-d)*w*1.2 …where d= day shift hrs w=day shift pay rate

putting x=55 from st1

d*w = (55-d)*w*1.2

At this point we know that we can get rid of w and calculate d. Thus st1+st2 are SUFF. Ans C

What if st2 instead gave some different percentage “Grace's gross pay for the hours she worked on day shift last month was exactly 40% of her total gross pay for last month.”

We can (Day Shift Pay)/40% = Total and (Night shift Pay)/60%= Total

=> d*w/0.4 = {(x-d)*w*1.2}/0.6

Similarly if st2 instead gave some crazy fraction “Grace's gross pay for the hours she worked on day shift last month was exactly 7/13 of her total gross pay for last month.”

We can (Day Shift Pay)/(7/13) = (Night shift Pay)/(6/13)

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