How to use convex hull for this problem Problem:

You are to collect a total of $N$ litres of red and $M$ litres of blue liquid. For doing job $i$ for time $t$, you get $a_i  t$ litres of red and $b_i  t$ litres of blue liquid. $t$ need not be an integer. At a time you can only do one job. You can choose some (or all) of the jobs and do it for your desired time. Given such $n$ jobs in the input and their respective $a_i$ and $b_i$ find the lowest total time in which you can get $N$ litres of red and $M$ litres of blue liquid.

How do I solve this problem using convex hull?
Perhaps you want a hint first?
Of course you start by considering all vectors $(a_i,b_i)$ first. The vector $(a_i,b_i)$ indicates the amounts you earn by working one hour on job $i$.

What is the meaning of any point on the line segment between points $(a_i,b_i)$ and $(a_j,b_j)$? 

If any point $(a_k,b_k)$ is within the triangle $(0,0)$, $(a_i,b_i)$, $(a_j,b_j)$ why can you discard it?