To form the sequence, begin with the term 7. Each subsequent term is equal to 6 added to the previous term, as follows:

,

the missing term.

The numbers in the given sequence increase in a uniform pattern, namely that half of the previous number is added to each successive number. Half of 33.75 is 16.88, added to 33.75 this gives us the correct answer: 50.63 (which was also the only answer that ended with the appropriate final decimal number.