Grade 4 Math Worksheets: Multiply 1 by 2-digit numbers - K5 Learning Mental multiplication worksheets: multiply 1-digit by 2-digit numbers. Below are six versions of our grade 4 math worksheet on multiplying 1-digit (1-9) by 2-digit (10-99) numbers. Students should try to figure out the answers mentally, regrouping in their head, without having to write down intermediary steps. These worksheets are pdf files.