Question

two numbers are randomly chosen from 1, 3, 5, 7, ..., 145, 147, 149

Answer 1

in how many ways can they be chosen so that we have their product divisible by 5?

Answer 2

Total numbers: 75.
Numbers divisible by 5: 15.

Case-I: One number and only one number of the two chosen ones is divisible by 5.
\(15\) numbers divisible by 5 and \(60 \) numbers that are not. Thus there are \(15\cdot 60= 900\) such pairs.

Case-II: Both numbers divisible by 5.\[\binom{15}{2}=105\]Total numbers: 1005.

Answer 3

The given answer is 1020. Am I missing something?

Answer 4

\(\color{white}{bookmark}\)

Answer 5

oooh, goddammit. you know what, the original question has a list that goes up to 151 :P

Answer 6

in that case, divisible by 5: 15... total: 76

Answer 7

so I'm right in a way. I just read the list till 149 lol