45% is just about half. So, you know right off he has about 9km left to go. (This is just for a sanity check)

So, the whole walk is 9/.45 = 20km

20-9 = 11 km left to go

The key idea here is figuring the total distance if you know some of it and the % that represents.

If you want to know what to divide by what, think of something easy.

If 9 kn were half the distance, the whole would be 18km, right?

That is 9 / (1/2) = 9*2 = 18

But you only have 45%, or .45

So, divide by .45 -- you expect a number a little more than 18.

Knowing a part and the fraction it represents, just divide the part by the fraction. Play around with some values till you feel comfortable with the idea.