Is this number divisible by 2? Half the time the number is, half the time it isn't. Steps give the divisibility test.

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A number is divisible by $4$ if and only if the last two digits form a number that is divisible by $4$.

\nOf course, this means you need to either know all the multiples of $4$ up to $100$ (unlikely) or be willing to divide the two digit number by $4$ (which is as easy as halving then halving again!) and see if you get a whole number!

\nAlso useful is realising a number must be divisible by $2$ to have any chance of being divisible by $4$.

\n$\\var{num}$ ends with the digits $\\var{d1}\\var{d0}$, which is NOT divisible by $4$ and so $\\var{num}$ is NOT divisible by $4$.

\nHow do we know $\\var{d1}\\var{d0}$ isn't divisible by $4$? Well, to be divisible by $4$ a number has to be divisible by $2$ (which means it has to end in a $0,\\,2,\\,4,\\,6$ or $8$) but $\\var{d1}\\var{d0}$ ends in a $\\var{d0}$ so it isn't even divisible by $2$ let alone $4$.

\nHow do we know $\\var{d1}\\var{d0}$ isn't divisible by $4$? Well, if we halve $\\var{d1}\\var{d0}$ we get $\\var{h}$. But if we try to halve $\\var{h}$ we will not get a whole number and so $\\var{d1}\\var{d0}$ is not divisible by $4$.

\n$\\var{num}$ ends with the digits $\\var{d1}\\var{d0}$ which is divisible by $4$ and so $\\var{num}$ is divisible by $4$.

\nHow do we know $\\var{d1}\\var{d0}$ is divisible by $4$? Well, if we halve $\\var{d1}\\var{d0}$ we get $\\var{h}$ and if we halve $\\var{h}$ we get $\\var{q}$ which is a whole number and so $\\var{d1}\\var{d0}$ is divisible by $4$.

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