The compound interest on a certain sum of money for 2 years

The compound interest on a certain sum of money for 2 years at 10% per annum is ₹ 420. The simple interest on the same sum at the same rate and for the same time will be

Given that , Compound Interest ( CI ) = ₹ 420 , Rate ( R ) = 10% , Time = 2 years
If the principal be P then

C.I. = P			1 +	R		^T	− 1
				100

⇒ 420 = P			1 +	10		²	− 1
				100

⇒ 420 = P		121 − 100
		100

⇒ 420 =	P × 21
	100

⇒ P =	420 × 100	= ₹ 2000
	21

∴ S.I. =	PRT
	100

S.I. =	2000 × 10 × 2	= ₹ 400
	100

We can find required answer with the help of given formula :
Here, C.I. = Rs. 420 , R = 10% , S.I. = ?

C.I.= S.I.		1 +	R
			200

420 = S.I.		1 +	10
			200

420 = S.I.		210
		200

S.I. =	420 × 200	= ₹ 400
	210