Math question:(

20000(1-30%)^t - 11000(1-20%)^t = 300020 * 0.7^t - 11 * 0.8^t = 3Let 0.7^x = 0.8 ,x = log0.8 / log 0.7 = 0.625621606So 0.7^0.625621606 = 0.8
i.e. (0.7^t)^0.625621606 = 0.8^tLet 0.7^t be k ,20k - 11k^0.625621606 = 311k^0.625621606 - 20k + 3 = 0It is no formula to solve it.

Make use of computer :http://www.wolframalpha.com/input/?i=20k+-+11k%5E0.625621606+%3D+3k = 0.511652131067618...

0.7^t = 0.511652131067618

t = log 0.511652131067618 / log 0.7

t = 1.87877...