The standard way to solve this is with algebra (as Jomo did - but check for a sign error!); but since you put this under Arithmetic, I'll suppose that algebra is not available. (Please let us know -- that's one of the things we request in our
submission guidelines.)
I like a visual method at that level. The first fact can be represented as
Code:
+---------+....+
M:| | 60 :
+---------+....+
+-------------------+
Y:| : 60 |
+-------------------+
which shows that Yati has $120 more than Minah.
You can do something similar to see that in the second case, Yati would exceed Minah by $560. What multiple of Minah's new amount would this $560 be?
There are also various trial and error methods that make sense for developing a number sense in arithmetic.
But if your son knows any algebra, that will take a lot less thinking!