If 1 mile = 1.609 km then we can use it as the conversion factor in the following way, remembering that the equation has to be set so that the miles cancel with miles.
Well x=1 always unless it tells you other wise so 1+0.5 would equal 1.5 and so after that you keep adding so 1.5 + 0.5 equal 2 so you keep taking the previous answer and adding 0.5 to it