Maths prove it

My maths goal was make a pattern and give a rule for general term in words/symbols. 
My solo level is extended abstract because I know how to do it and I could help others when they need help.
My question was Tina and Tom were twins. Tina saves and Tom spends. Tom found a $20 note on Sunday evening and spent $2 a day starting on Monday. Coincidentally, Tina started work that Monday and gets $2.50 a day. How long till Tina has more money than Tom?
This is my answer for the question is in the picture below.
My next step is to have a teacher conference and find a new goal.