Answer: multiply each by 1.5 to get the next term.
Discussion: I noticed that 7.5 is 1.5 times 5 -- and 5 is the first number in the sequence. I wondered if that was the pattern so I used Calc to multiply 7.5 by 1.5 and got 11.25. (Looking good). Then I multipled 11.25 by 1.5 and got 16.875.
