Answer:
This type of problems has restrictions which are represented by regions in the coordinated system.
In this case, variables are Practicing Piano (x) and Computer Time (y). So, the range of time that Martha has available to use is 50 minutes, because after then she's having dinner and the problem says that she want to spend some time before dinner. So, the restriction for both variable x and y is 50 minutes, which means that graphically, the line has to have 50 minutes in common in both axis, as it's shown in option C.
Then, if we read carefully, we find that Practicing Piano has a restriction of "at least 20 minutes", meaning that Martha was to spend 20 or more minutes Practicing Piano. So, the line that cuts 20 in x-axis have to be coloured to the right, representing she spending 20 or more than 20 minutes, as it's shown in option C.
Therefore, the right graph is C, because it fits perfectly with the problem restrictions.