I ran across the problem below and had some trouble with it. I know i can solve it, but it involves tons of multiplication and hte use of the quadratic formula...Is there a quicker way (without resorting to back solving) ?
A rectangular yard is 20 yards wide and 40 yards long. It is surrounded by a thick hedge that grows on the boarder of the property, but comkpletely within the boundaries of the yard. If the hedge covers and area of 171yards^2, what is the width?
a. 160/120
b. 171/120
c. 180/120
d. 191/120
e. 800/120
(thanks!)
A quick way to solve this problem
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 202
- Joined: Tue Sep 08, 2009 11:34 pm
- Thanked: 15 times
- GMAT Score:760
Well, I got this equation:
171 = 120x - 4x^2
x being the width of the hedge.
This tells me that it can't be (A) or (B), because x > 171/120 in order to be able to subtract that 4x^2 and still equal 171. (E) seems absurd to me, so if I encountered this question, I'd be at a 50/50 guess between C & D.
I think at this point I'd backsolve. I'll try (C) since it's much cleaner. The nice thing is that the 120 term would cancel out, so you'd be left with
171 = 180 - 4*(180/120)^2
3/2^2 --> 9/4 * 4 --> 36/4 --> 9. 180 - 9 = 171.
(C) is our answer. If this had not been the correct answer, then I would've gone with (D) by elimination.
BTW: Backsolving would've been an excellent strategy here. You would've gotten the correct answer right away (i.e. C) once you got the quadratic equation.
171 = 120x - 4x^2
x being the width of the hedge.
This tells me that it can't be (A) or (B), because x > 171/120 in order to be able to subtract that 4x^2 and still equal 171. (E) seems absurd to me, so if I encountered this question, I'd be at a 50/50 guess between C & D.
I think at this point I'd backsolve. I'll try (C) since it's much cleaner. The nice thing is that the 120 term would cancel out, so you'd be left with
171 = 180 - 4*(180/120)^2
3/2^2 --> 9/4 * 4 --> 36/4 --> 9. 180 - 9 = 171.
(C) is our answer. If this had not been the correct answer, then I would've gone with (D) by elimination.
BTW: Backsolving would've been an excellent strategy here. You would've gotten the correct answer right away (i.e. C) once you got the quadratic equation.
-
- Legendary Member
- Posts: 882
- Joined: Fri Feb 20, 2009 2:57 pm
- Thanked: 15 times
- Followed by:1 members
- GMAT Score:690
Can you explain how you did you get the quadratic? TxWell, I got this equation:
171 = 120x - 4x^2
- papgust
- Community Manager
- Posts: 1537
- Joined: Mon Aug 10, 2009 6:10 pm
- Thanked: 653 times
- Followed by:252 members
Sanjana,sanjana wrote: Shouldnt it be 171=120x+4x^2?
I guess 171=120x-4x^2 is right. See the words "hedge completely within boundaries of the field"
So it will be (40-2x)(20-2x).. not (40+2x)(20+2x)
Hope my reason is meaningful!