Im stuck on a natural log problem. I forget how to convert them. 400=e^3t

400 = e^(3t)   take the ln of each side

ln 400  = ln e^(3t)    and we can write

ln 400 = (3t) ln e       and ln e = 1  so we can disregard this.....so we have

ln 400 = 3t      divide both sides by 3

ln 400 / 3  = t = about 1.997