What sum of money will amount to Rs. 520 in 5 years and to Rs 568 in 7 years on simple interest?

Rs. 400

We know that,

Principal = Amount – simple interest

We have,

Amount in 5 years = Rs. 520

Amount in 7 years = Rs. 568

Simple interest for 2 years =  568 – 520 =  Rs. 48

Simple interest for 5 years = (48/2) × 5 =  Rs. 120

Principal =  520 – 120 =  Rs. 400