Diversify the questions in PS 27 and 28

[taliaferrojm]

Could be nice to have slightly different questions for these problem sets. As it was, many students didn't even think these were real because they were the exact same as what they saw in class.

[jayhesselberth]

@nmukherjee can you take a look and see if we could differentiate the problem set questions?

[nmukherjee]

Agreed - will do.