Help maths question I don't get it

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.     

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

$$\\\frac{8.6\times10^6 \times 4.24 \;seconds}{30000} = 1215\;\; seconds\qquad \mbox{(All the other units cancelled out)}\\\\ \frac{1min}{60sec}\times \frac{1215 sec}{1} = \frac{1215 min}{60} = 20.25\;minutes\\\\ \mbox{Therefore download time is approximately 20 minutes}$$