One simple method of finding out scrap value after 15 years is as shown below:
Cost of Machine = Rs.40000 and rate of depreciation is 10% on written down value method
Life of Machine = 15 years
Scrap of value after 15 years = Rs.40000 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 x .9 = Rs.8235.64
Note: After charging depreciation at the rate of 10%, written down value of machine is 90% i.e. .9.
Second method is by using the formula: R = [1 - Nā(scrap value/cost)] X 100 (N is life of machine in number of years and also it is Nth root)
10 = [1 - 15ā(S/40000)] X 100
10/100 = [1 - 15ā(S/40000)]
1 - .1 = (15āS/40000)
(.9)^15 = S/40000
S = Rs.8235.64