Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
(a) Step 1, i.e., 3 × 3 subgrid: in this example, the participants need to fill in the missing numbers (2 and 5); (b) step 2, i.e., 9 × 9 subgrid: in addition, the participants need to fill in missing number according to the row and column rules.
