Perhaps you mean t(3) given t(5) = 25 and t(7) = 100 however, there isn't enough information given to do that problem either (you would need to know what type function it is - like linear, exponential, quadratic, etc).

I'm guessing this is an geometric sequence and you are referring to term # 5 being 25 and term # 7 being 100. That would mean the term # 6 would need to be 50 and the common ratio would be 2 (if term 5 is 25 then term 6 is 2•25 = 50, and term 7 is 2•50 = 100).If so, term 4 would be 12.5 (25/2 = 12.5 and term 3 would be 6.25(12.5/2 = 6.25)

But you might want to give us the full question next time.