For part (ii), you could also solve it this way, since in part (iii) they're asking you for the answer in km square (so you don't have to convert the numbers a second time):

If 1cm = 500m, and 1km = 1000m,

500m = 500 x 1/1000 = 0.5 km

So now you have a ratio of 1cm : 0.5km

Which you can simplify to whole numbers by multiplying each number by 2, so the ratio now looks like:

2cm : 1 km

Therefore,

28 km ------> 28 x 2 = 56cm