a. 9 x 10 = 90.
90 two-digits numbers can be formed from the digits 0 1 2 3 4 5 6 7 8 9
b. 9 x 5 = 45
45 two-digit numbers are even.
c. 9 x 5 = 45
45 two digit numbers are odd.
d. 30 two-digit numbers are divisible by 3.
I suggest you to do the listing process.
e. 4 x 10 = 40.
You can answer it using tree diagram.
f. greater than 49 = 50
less than 70 = 69
there are two numbers so, 2 x 10 = 20