For the interpolation do 85/2 which gives 42.5, you don't need to do +1 because it's continuos. Then that gives 42.5, find in what class the 42.5 is which is in the 70-80 class. Then you'll notice up to that class there are 29 terms so do 42.5-29 giving 13.5. Now you need to find 13.5/25 of the way through 70-80 and add that to 70. This can be done by 70 +[(13.5)/25]x(80-70) giving you the answer of 75.4