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

Question

parthkohli · Answer

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

parthkohli · Answer

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.

parthkohli · Answer

The given answer is 1020. Am I missing something?

mathmath333 · Answer

$\color{white}{bookmark}$

parthkohli · Answer

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

parthkohli · Answer

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

parthkohli · Answer

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