Could you show me the equation and solution in this time word problem? Thanks

Question

How can I be able to derive the correct answer in this word problem:

One of the two watches gain 12 seconds in 18 days and the other loses 20 seconds in 30 days. If the watches were together at noon on a certain day, how many minutes apart would they be 90 days later?

Answer 1

the watch that gains 12 sec/18 days:

 

12 (90 / 18) = 60 seconds gained

 

the watch that loses 20 sec/30 days:

 

20 (90 / 30) = 60 seconds lost

 

One gained 60 seconds and one lost 

60 seconds, together they would be

120 seconds apart or 2 minutes apart