Official Solution:


(1) Half of the computers that are equipped with built-in modems do not have infrared ports. Look at the double set matrix below:



As you can see we can not get the value of the yellow box. Not sufficient.

To elaborate more on this statement: since half of the computers that are equipped with built-in modems do not have infra-red ports, then another half of computers that are equipped with built-in modems DO have infrared ports. So, two boxes in "Modem" column are equal, since stem says that # of computers with modems and infra-red ports is 25 then # 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. Look at the double set matrix below:



As you can see we can not get the value of the yellow box. Not sufficient.

(1)+(2) When we consider two statements together we get the following matrix:



So, we have that 15% of the computers are not equipped with either of these devices.


Answer: C
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For (1) first we got that # of computers with modems is 50 (see explanation for that), thus # of computers without modems is also 50.
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

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from 1) its only saying half of computers with built in modems do not have infrared ports. how its conveying computers with modem are 50%?

How do we solve this with formula?