Kevin had to measure the diameter of a hair strand and the diameter of a grain of sand for his science experiment. he found the diameter of the hair strand to be 0.00006 inches and the diameter of a grain of sand to be 1.3 × 10-3 inches.

This is the concept of linear scale factors. 
Diameter of hair strand is 0.00006=6×10^-5 inches
Diameter of sand grain is 1.3x10^-3 inches
Thus the the number of times the hair sand grain is bigger than hair strand will be:
linear scale factor=(diameter of sand grain)/(diameter of hair strand)
=(1.3×10^-3)/(6×10^-5)
=21 2/3 times

The answer is 6 weeks