₹18000 is lent at simple interest — at 6% per annum for the first 2 years, at 9% per annum for the next 3 years and at 12% per annum thereafter. The total interest at the end of 8 years (in ₹) is:

Handle each slab separately: $18000\times 0.06\times 2=$ ₹2160, $18000\times 0.09\times 3=$ ₹4860 and $18000\times 0.12\times 3=$ ₹6480. Total = 2160 + 4860 + 6480 = ₹13500. (Running the first rate for all 8 years gives ₹8640, the last rate for all 8 years gives ₹17280, and averaging the three rates to 9% gives ₹12960 — wrong, because the slabs cover 2, 3 and 3 years, not equal spells.)