The Question clearly states that 3% more produces 720 for 2 years
So 3% produces 720/2 for 1 year
which is 360 per year
Interest= P*T*R/100
360 = P*1*3 / 100
On Solving P=360*100/3
P=12,000
You can also solve like this,
3% = 360
!00%(Prinncipal) =?
100% = (360/3)*100 = 12000