Answer:
45 numbers
Step-by-step explanation:
The 10-digit numbers are 10-99. We can manually write them out to start and see if we can find a pattern.
We can start 10-19: The numbers that have one odd digit and one even digit at 10, 12, 14, 16, 18 (since 1 is odd, we can only pick the numbers that have an even unit digit)
Next, we move on to 20-29: The numbers that follow the property are 21, 23, 25, 27, 29
Both of these examples have 5 numbers that follow the property. We can notice that all of the subsequent clusters (30-39, 40-49, 50-59, 60-69, 70-79, 80-89, 90-99) will also have 5 each. The numbers that start with an odd number will have the same unit digits as the 10-19 cluster, and the numbers that start with an even number will have the same unit digits as the 20-29 cluster. Observe:
30-39: 30, 32, 34, 36, 38
40-49: 41, 43, 45, 47, 49
50-59: 50, 52, 54, 56, 58
60-69: 61, 63, 65, 67, 69
70-79: 70, 72, 74, 76, 78
80-89: 81, 83, 85, 87, 89
90-99: 90, 92, 94, 96, 98
In all, there are 9 clusters of numbers, and each one has 5 numbers that follow the property. Therefore, the total is 9*5 or 45 numbers
