Answer:
81mm
Step-by-step explanation:
the number of steps is small enough that we can simply list out each step
let the time elapsed be t and the original length be L ( = 1 mm)
when t = 0, L = 1mm
when t = 10, L = 3mm
when t = 20, L = 3 x 3 = 9mm
when t = 30, L = 9 x 3 = 27mm
when t = 40, L = 27 x 3 = 81mm
