Answer:
45
Step-by-step explanation:
There are 90 two-digit numbers (10-99), Numbers using 1 include 10-19, numbers including 3 are 30-39, and numbers including 5 are 50-59. This amounts to a total of 27. Since the numbers are also in other two-digit numbers such as 21, we have 21,23,25,41,43,45,61,63,65,71,73,75,81,83,85,91,93,95, which amounts to a total of 18.
18+27= 45.
So there are 45 double-digit numbers using 1, 3, and 5.
