The three-digit number "ab5" is divisible by 3. How many different three-digit numbers can "ab5" represent?

30

Step-by-step explanation:

I can't prove this, but I can tell you the answer.

30.

a = 100 s  digit

b = 10 s digit

5 is always the units digit.

I'm pretty sure that 30 is the correct answer.

What an interesting question. If I find out how it is done, I'll post it in the comments.

I can say this much.

a + b + 5 must be divisible by 3.

a = 1     b = 9     + 5

a = 1     b = 6     + 5

a = 1     b = 3     + 5

a = 1     b = 0 +  5

Three more are obtained by interchanging the 100s and 10s digit except for b = 0

a = 9     b=1    + 5

a = 6     b =1   + 5

a = 3     b= 1   + 5

a = 2    b = 2 + 5

a = 2    b = 5  + 5

a = 2    b = 8  + 5

only 2 more are obtained by interchanging the 10s and 100s digit

a = 3   b = 4    +5

a = 3   b = 7    +5

You can't go any further. You get 2 more by interchanging the b and a digits.

I hope you see what I've done here. I don't think this is an exact proof, but it is an exhaustive search.