We can find the area of the doormat, in square meters, by multiplying 1/2 and 3/10, getting 3/20. Each square is 1/10*1/10=1/100 square meters.
(3/20)/(1/100) will give the ratio of the size of the doormat to the size of the squares, and thus the amount of squares needed. This is equal to 15 squares, so the doormat will be divided into 15 squares.