Answer:
considering it was simple interest,
We have the formula:
 I = PRT ÷ 100
where,
- I = Interest
 - P = principal
 - T = time (in years)
 - R = rate
 
for the first part of the question, 
Thus, I = (2160 × 20 × 4)÷ 100
I = Rs. 1728
Now, for the second part,
- P = Rs. 1800
 - I = 1728
 - R = 12
 
T = 100×I÷PR
T = 100 × 1728÷(1800 × 12)
T = 8 Years