solutions to combinatorics exercises

Your answer is wrong (to the problem of, how many 2-digit numbers have the digit of tens strictly smaller than the digit of units)

#Combinatorics

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Math	Appears as
Remember to wrap math in `$` ... `$` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$

Comments

If you believe that a practice problem is wrong, you can report it directly and we will take a look at it.

I do not see a question of "how many 2-digit numbers have the digit of tens strictly smaller than the digit of units" in the practice section. The closest question that I see is "How many 2 digit numbers are there, such that the units digit is strictly smaller than the tens digit?". These 2 questions would have different answers.

Calvin Lin Staff - 6 years, 4 months ago

Yes, you are correct. 'My' question has the answer of 36, your question has the answer of 45. My recollection of the exact formulation was wrong (generally I work on these problems in my head, i.e.., without paper, pen or repeated looking on the screen -- so, it's probably bound to happen every now and then...)

Predrag Tosic - 6 years, 4 months ago

Great answers :)

Do you have any feedback on your usage of Brilliant? Is there any thing that you would like us to improve on?

Calvin Lin Staff - 6 years, 4 months ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$