2. Variant 1. Find the number of four-digit numbers where the digit in the units place is exactly 1 more than the digit in the tens place (the number cannot start with zero).
Answer: 810.

Solution. The leading digit of the number can be chosen in 9 ways (any digit except zero). The digit in the hundreds place can be chosen in 10 ways (any digit will do). The digit in the tens place can be any digit from 0 to 8, and the digit in the units place is uniquely determined by the chosen digit in the tens place. In total, there are $9 \cdot 10 \cdot 9=810$ options.