Official Solution:In a certain office, 25% of the computers are equipped with both built-in modems and infrared ports. What percentage of the computers are not equipped with either of these devices? (1) Half of the computers equipped with built-in modems do not have infrared ports.
Refer to the double set matrix below:
From the matrix, it is clear we cannot determine the value in the yellow box. Not sufficient.
To elaborate more on this statement: since
half of the computers equipped with built-in modems do not have infrared ports, the other
half of computers that are equipped with built-in modems DO have infrared ports. Thus, two boxes in the "Modem" column are equal; since the stem says that
the number of computers with modems and infrared ports is 25%, then
the number of computers with modems but without infrared ports is also 25%.
(2) 35% of all computers are equipped with infrared ports but do not have built-in modems.
Refer to the double set matrix below:
From this matrix as well, we are unable to determine the value in the yellow box. Not sufficient.
(1)+(2) When we consider both statements together, we get the following matrix:
Thus, we can conclude that 15% of the computers are not equipped with either of these devices. Sufficient.
Answer: C