
borrowing problem
X borrowed a certain sum from Y at a certain rate of simple interest for 2years. He lent this sum to Z at the same rate of interest compounded annually for the same period. At the end of two years he received $4200 as compound interest but paid $4000 only as simple interest. Find the sum and rate of interest.

Do you have any formulas?
I = Prt  for example.
M = The Certain sum of money
Simple
$\displaystyle M \cdot r \cdot 2 = 4000$
Compound
$\displaystyle
M \cdot (1+r)^{2}M = 4200
$