Find the unit rate with the second given unit in the denominator.
4200 words in 60 minutes
a. 60 min/word
b. 76 2/3 min/word
c. 76 2/3 word/min
d. 4600 word/min

First, the unit is words per minute, not the other way round because you're interested in how fast people can type, not how much time passes for every number of words they write or read (that would be minutes per word)

now, we know they do 4200 in 60 minutes. 

we need to divide 4200 by 60- first,

so the answer is, 70 words per minute

I think you made a typo in your answer and you meant 4600 instead of 4200 - then the answer is C. 76 2/3 word/min