Then 14 is16 mm.

Then 13 day is 8 mm

So 13 is 8mm long.

Calculation formula:

32/8=4; Calculate how many times the length of a day 15 is 8 mm.

4/2=2; Divide the above multiple by the daily increase multiple to get the number of days from 8mm to 32 mm.

15-2= 13 (days)