Let the book value of stock be x.
Romesh took over the stock at 10% less than book value, which means he took it at 90% of the book value.
Therefore:
90% of x = Rs. 8,100
10090×x=8,100
x=8,100×90100
x=90810,000
x=9,000
Verification: 10% of Rs. 9,000 = Rs. 900
Book value - 10% = Rs. 9,000 - Rs. 900 = Rs. 8,100 ✓
The book value of the stock was Rs. 9,000.