I always use the units to help me do rates.
My method is quite unique I believe but when you get the hang of it it makes working out any rates related questions really easy - even really complicated looking ones.
I have explained this one in detail so it looks long but I normally would do it in 3 easy lines. 
I think it is worth puting the effort in and working out what I have done.

$$\\\frac{30000kb}{1sec}\qquad \mbox{this can also be expressed as }\qquad \frac{1sec}{30000kb}\\\\
\mbox{Alex wants to download 4.24GB }\\
\mbox{(I have to assume this because it is not written into the question)}\\\\
\mbox{There are } \frac{8.6\times 10^6kb}{1GB}\quad \mbox {and Alex want to know seconds/download}\\\\\\
\mbox{Start with the ratio that has seconds on the top}\\\\
\frac{1sec}{30000kb}\\\\
\mbox{Now look for the one with kb on the top because we want to cancel those out}\\\\
\frac{1sec}{30000kb} \times \frac{8.6\times 10^6kb}{1GB}\\\\
\mbox{Now look for the one with GB on the top because we want to cancel those out}\\\\
\frac{1sec}{30000kb} \times \frac{8.6\times 10^6kb}{1GB}\times \frac{4.24GB}{1} \\\\
\mbox{So this will leave us with } \\\\$$
$$\\\frac{8.6\times10^6 \times 4.24 \;seconds}{30000} = 1215\;\; seconds\qquad \mbox{(All the other units cancelled out)}\\\\
\frac{1min}{60sec}\times \frac{1215 sec}{1} = \frac{1215 min}{60} = 20.25\;minutes\\\\
\mbox{Therefore download time is approximately 20 minutes}$$
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