A man jumps from above - his speed of descent is given by V = 50(1-2^-0.2t) m/s where t is the time in seconds. Find the time taken for his speed to reach 40 m/s.

I know to set the equation to:

40 = 50(1 - 2^-0.2t)

but i can't seem to carry it out from there..because i can't take the log of both sides when the numbers are negative.