So 1.70 (you can multiply it by 100 to make things easier if you want) - 1.50 = 0.15. So 0.15 is the difference between one term and the next.
Also, if you subtract 1.40 from 1.70 you'll find that the difference is 0.30, which is twice 0.15. And 1.70 is two terms away from 1.40. Thus, 1.40 + 0.15m is true, (m being the amount of terms away from the starting number.) In the case I mentioned, 1.40 + 0.15(2) was true. So, plug 31 into the equation:
1.40 + (31)(0.15) = 6.05. This is your answer.