Let the money added be ₹ P, Then,
[(4800 + P) x 12 x 3]/100 - (4800 x 9 x 3)/100 = 720
⇒ (4800 + P) x 36 - (4800 x 27) = 720 x 100
⇒ (4800 + P) x 4 - (4800 x 3) = 8000
⇒ 4800 + P - (1200 x 3 ) = 2000
⇒ P + 1200 = 2000
∴ P = 800
So, money added is ₹ 800.