It would take 21 minutes to run the 42 blocks remaining
Step-by-step explanation:
If Susan can run 2 city blocks per minute and she already ran 18 blocks of the 60, it means that she has to run 60 minus 18 blocks: that is 42 blocks in total. at a rate of 2 per minute it would take her 42 over that same rate: that's minutes in total.