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The double factorial, denoted by $n!!$, returns the product of all of the odd integers that are less than or equal to $n$. For example, $7!! = 7 * 5 *3 * 1$. What is the units digit of $1!! + 3!! + 5!! + 7!! + ... + 49!!$?

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well, you know that n!! ends in 5 for all n >= 5
There are (49+1)/2 = 25 terms in your sum. The last 23 of them end in 5.
So, what do you think?
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