Should the accuracy calculation in Chapter 4 be weighted?

[hitsvilleusa]

The line in chapter 4 when computing the accuracy is:

accuracy_3s,accuracy_7s,(accuracy_3s+accuracy_7s)/2

Is /2 appropriate here? It would seem that the accuracy should take into account the number of images in the validation set? Granted these are pretty close in this case, but it would seem that this would make a big difference if the number of images in these two sets were quite different?