Quoting Khong Pek Mao’s answer below:
This is a Matrix question.

Save 
Spent 
Total 

Sarah 
1u 
120 
1u + 120 

Jonathan 
1.9u 
48 
1.9u + 48 

1u + 120 = 1.9u + 48 + 0.3u
120 – 48 = 2.2u – 1u
1.2u = 72
1u = 60 days
b) 120 / 60 = $2
Unquote
Hi Khong, how do you get “1.9u”? If it is because Jonathan saves 90cent more, this info is not equivalent to 1.9u savings by Jonathan, where Sarah saves “1u” and “u” is the no. of days.
Let’s say Sarah saves $x per day, the summary table should be as below:

Save 
Spent 
Total allowances 

Sarah 
($x) u days 
120 
xu + 120 

Jonathan 
($x+0.9) u days 
48 
xu + 0.9u + 48 

xu + 120 = xu + 0.9u + 48 + 0.3u
[where 0.3u is the extra allowances to “add to J” so the allowances equate)
120 – 48 = 1.2u [where xu are cancelled out]
1.2u = 72
1u = 60 days
b) 120 / 60 = $2
So in this sense, Sarah’s daily savings ($x) or Jonathan’s daily savings ($x+$0.90) is not important in the solution (question do not have the information to solve this $x).
Key deduction from the problem is that Jonathan is spending $1.20 less than Sarah per day… because he saves $0.90 more (=spend $0.90 less) but also need to “save up” the $0.30 difference (=spend $0.30 less) as their “unfair” mother is giving $0.30 more to Sarah each day. And then, applying this daily difference to the total difference in their accumulated spendings like Ender’s workings.