Hi

I have to solve this problem:

ABC Co is looking into promoting sales of their shares, which are expected to result in extra sales. After the promotion, the extra

sales reach 4000 and will then start decaying at a rate of −800e^(−0.2t) , where t is time in days after the promotion campaign. How many extra sales are obtained 40 days after the close of the campaign?

Please advise!

Many thanks

ola