Take an example
You decided to go for 6 months instalment plan
Advance = 7,000
monthly instalment = 12,450
after six month you paid = 12,450 x 6 = 74,700
Total paid = 74,700 + 7000 = 81,700
Over paid in six months = 81,700 - 78,400 (actual price) = 3,300
So you paid Rs 3,300 extra in six months
In this way you can calculate rest instalment plans