## Online Course Discussion Forum

### MC II-A HW help

Hello!

I don't really get how to do another proof for 1.24 and I'm not sure how to start 1.28 and 1.29.

Thanks!

For 24, answering this questions might help: How many $2$-digit numbers are there? If you look at the numbers in order, how often does a multiple of $9$ appear? Just by looking at the digits of a number, how can you decide if it is a multiple of $9$?

On 28 complementary counting will help. How many $7$-letter words with no two consecutive letters are there? How many of those DO NOT start with two consonants? (You might want to split in cases to answer the second question.)

On 29 recall a rook attacks all squares that are in its same row or column. Can two rooks be on the same edge?

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