11. Masha put a multiplication sign between the second and third digits of the number 2020 and noted that the resulting product $20 \times 20$ was a square number.
How many integers between 2010 and 2099 (including 2020) have the same property?
A 1
B 2
C 3
D 4
E 5
SolUTION
$\mathbf{C}$

Each number begins with 20 , and $20=5 \times 2^{2}$. Hence, to make a square product, the last two digits must make a number which is a product of 5 and a square number. The possibilities between 10 and 99 are $5 \times 2^{2}=20,5 \times 3^{2}=45$ and $5 \times 4^{2}=80$. Therefore there are three possible numbers: $2020,2045,2080$.