Tricky: If AB || DE, and AB = 10, what is the length of DE?
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If AB || DE, and AB = 10, what is the length of DE?
(1) BC = 6 and CE = 4
(2) DC = 3 and CB = 6
Answer: B
Difficulty level: 650 - 700
Source: www.gmatprepnow.com
GMAT/MBA Expert
- Brent@GMATPrepNow
- GMAT Instructor
- Posts: 16207
- Joined: Mon Dec 08, 2008 6:26 pm
- Location: Vancouver, BC
- Thanked: 5254 times
- Followed by:1268 members
- GMAT Score:770
Timer
00:00
Your Answer
A
B
C
D
E
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Let's first add the given information (and their implications) to the diagramBrent@GMATPrepNow wrote:
If AB || DE, and AB = 10, what is the length of DE?
(1) BC = 6 and CE = 4
(2) DC = 3 and CB = 6
Answer: B
Difficulty level: 650 - 700
Source: www.gmatprepnow.com
If AB || DE, we know that ∠BAC = ∠CED and ∠ABC = ∠DCE
Also, since vertically opposite angles are equal, we know that ∠ACB = ∠DCE
Now that we've identified 3 pairs of EQUAL angles, we can conclude that the two triangles are SIMILAR
So, we need to determine which sides are corresponding.
Notice that side AB and side DE BOTH lie between the angles denoted by a star and a square.
This means that side AB corresponds with side DE (denoted by BLUE lines)
Likewise, side BC and side CD both lie between the angles denoted by a circle and a square.
This means that side BC corresponds with side CD (denoted by RED lines)
Finally, side AC and side CE both lie between the angles denoted by a circle and a star.
This means that side AC corresponds with side CE (denoted by GREEN lines)
Now onto the question!!!!
Target question: What is the length of DE?
Statement 1: BC = 6 and CE = 4
Add this info to the diagram:
Notice that we don't know the lengths of two corresponding sides.
As such, we can't determine the magnification factor of the two similar triangles (see the video below for more information about magnification factor)
As such, there's no way to determine the length of side DE
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: DC = 3 and CB = 6
Add this info to the diagram:
Notice that DC and CB are corresponding sides.
So, the magnification factor of the similar triangles = 6/3 = 2
In other words, ∆ABC is 2 times the size of ∆EDC
So, if side AB = 10, then side DE (the side that corresponds with side AB) must have length 5
Since we can answer the target question with certainty, statement 2 is SUFFICIENT
Answer: B
RELATED VIDEO FROM OUR COURSE
https://www.youtube.com/watch?v=c2sNcDDhUSQ