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Is this number divisible by 5? Half the time the number is, half the time it isn't. Steps give the divisibility test and a way to determine the number of fives.

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A number is divisible by $5$ if and only if it ends in the digit $0$ or the digit $5$. 

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Since $\\var{num}$ ends with the digit $\\var{mod(num,10)}$ it is NOT divisible by $5$.

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Since $\\var{num}$ ends with the digit $\\var{mod(num,10)}$ it is divisible by $5$.

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To work out how many fives are in a number, you can double the number and then cross off the end zero digit (this works because multiplying by $2$ and then dividing by $10$ is equivalent to dividing by $5$).

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For instance, $\\var{num}$ doubles to give $\\var{2*num}$ so we can cross of the zero and conclude there are $\\var{thismany}$ fives in $\\var{num}$. In other words, $\\frac{\\var{num}}{5}=\\var{thismany}$. 

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Is $\\var{num}$ divisible by $5$?

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