The percent difference between two numbers
and
is given by

The absolute value is there because we only care about the absolute percent difference, and not taking into account whether we go from
to
or vice versa. If we remove them, we have two possible interpretations of percent difference.
For example, the (absolute) percent difference between 3 and 6 is

In other words, we add 100% of 3 to 3 to end up with 6. This is the same as the percent difference going from 3 to 6. On the other hand, the percent difference going from 6 to 3 is

which is to say, we take away 50% of 6 away from 6 to end up with 3.
"Make comparisons to object measurements" tells us that the differences should be computed relative to "measurements for object". In other words, take
from the left column and
from the right column.



Most likely, the light wave will be absorbed by the wall. Without any information as to the size and color of the wall, the location and size of the hole, or the location of the light wave, this is a generalized probability problem. For all of the places the light could be, it's more likely that it hits the wall than the hole (if the hole is less than 50% of the area of the wall).