every 20m along a street there are lamppost from beginning to end.
a) the street is 800m long. How many lamppost are there on the street?
b) how many 20m spaces are there along the street?
c) Frankie lives between the lampost 21 and 22. How far from the beginning of the road does his property begin?
40 spaces, so 41 poles.
I think you can gt (c). Figure out how far the nth pole is.