For Nitin,
Total outstanding = ₹ 72,000
Nitin paid in cash= ₹ 22,000
Remaining dues = ₹ 50,000
Now, on this ₹ 50,000 we have to calculate interest @ 18% for 4 months
I = \(\frac{PRN}{100}\)
= 50,000 × \(\frac{18}{100}\times\frac{4}{12}\)
= ₹ 3,000
So, amount of new bill = Remaining dues + Interest
= 50,000 + 3,000
= ₹ 53,000