Susan can run 2 city blocks per minutes. She wants to run 60 blocks. How long will it take her to finish if she already run 18 blocks?

Well Susan has run 18 blocks

So every 2 blocks is 1 minute

2 x 9 = 18 so far 9 minutes

Now the remainder is 42 blocks

20 blocks is 10 minutes

20 x 2 = 40 blocks in 20 minutes

Then the last 2 blocks is 1 minute so

20 + 1 = 21 minutes