Step 1:
Use the formula for SI and substitute SI = 420 , R = 8 and T = 1.5
=> 420 = P x 8 x 1.5/100
=> 420 = 12P/100
Step 2:
Solve for P
420 x 100 = 12P ( Multiply both sides by 100)
35 x 100 = P ( Divide both sides by 12)
3500 = P
Principal of Rs 3500 gets a simple interest of Rs 420 in 1.5 years at 8% per annum.