How many Naya mixer-grinder disposed off by end of 2000 ?

Answer: Option B

In the year 1998, (75 - 25) = 50 mixer-grinder purchased.
Then, In 2000, 20% of 50 disposed off = 10.
Similarly, number of mixer grinder purchased in 1997 is 25.
Number of mixer-grinder disposed in 1999 = 20% of 25 = 5.
Total number of mixer-grinder disposed off at end of 2000
= 10 + 5
= 15