It is given that
CI = 5712
Rate of interest (r) = 4% p.a.
Period (n) = 2 years
We know that
A = $$P(1 + r/100)^{n}$$
It can be written as
CI= A - P = $$P(1 + r/100)^{n}$$ - P
=$$P[(1 + r/100)^{n}$$ - 1]
Substituting the values
$$5712 = P[( 1 + 4/100)^{2} - 1]$$
= $$P[(26/25)^{2} - 1]$$
By further calculation
= $$P[(676/625 - 1]$$
Taking LCM
=$$P[(676 - 625)/625]$$
=$$ P \times 51/625$$
Here
P= $$5712 \times 625/51$$
= $$112 \times 625$$
$$= 70000$$
Hence, the principal amount is 70000.