How can I calculate the following things in my head?
$_9C_4$ I know this is $\frac{9\cdot 8\cdot 7\cdot 6}{4\cdot 3\cdot 2\cdot 1}$ and then $3\cdot 2\cdot 3\cdot 7$ but I can't immediately come up with that calculation. Are there some quicker ways do this?
$6^5$
$68\cdot 27$
I don't want to rely on my calculator anymore. Can anyone give me some mental tricks?