有關directed number課題中的列式問題

八達通內有10元，買文具用去28元，餘下多少元？
a. (+10)-(+28)
b. 10-28
c. (+10)-28
ans. b
八達通內有-5元，增值50元後現有多少元?
a. (-5)+(+50)
b. -5+50
c. (-5)+50
ans. b
because when the negative number is at the beginning, you don't need to add brackets