Forum Home > GMAT > Quantitative > Problem Solving (PS)
Events & Promotions
|
|
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Dec 1511:00 AM IST
-01:00 PM IST
Attend this free GMAT Algebra Webinar and learn how to master the most challenging Inequalities and Absolute Value problems with ease.
Dec 1608:00 AM PST
-11:59 PM PST
GMAT Club 12 Days of Christmas is a 4th Annual GMAT Club Winter Competition based on solving questions. This is the Winter GMAT competition on GMAT Club with an amazing opportunity to win over $40,000 worth of prizes!- Dec 17
09:00 AM PST
-10:00 AM PST
Join Manhattan Prep instructor Whitney Garner for a fun—and thorough—review of logic-based (non-math) problems, with a particular emphasis on Data Sufficiency and Two-Parts. - Dec 18
10:00 AM PST
-11:00 AM PST
Here is the essential guide to securing scholarships as an MBA student! In this video, we explore the various types of scholarships available, including need-based and merit-based options.
01 Feb 2021, 07:51
Kudos
Bookmarks
D
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
95%
(hard)
Question Stats:
5% (01:39) correct
95%
(01:11)
wrong
based on 104
sessions
History
Date
Time
Result
Not Attempted Yet
What is the probability that all 6 coins tossed together will turn up heads, if it is known that 2 of them have turned up Heads?
A) 1/4
B) 1/16
C) 1/64
D) 1/57
E) 7/57
A) 1/4
B) 1/16
C) 1/64
D) 1/57
E) 7/57
26 May 2022, 14:52
Kudos
Bookmarks
doomedcat
What is the probability that all 6 coins tossed together will turn up heads, if it is known that 2 of them have turned up Heads?
A) 1/4
B) 1/16
C) 1/64
D) 1/57
E) 7/57
A) 1/4
B) 1/16
C) 1/64
D) 1/57
E) 7/57
How about an easier way?!?!
How many total possible outcomes are there if we toss six coins? 2*2*2*2*2*2 = 2^6 = 64
But are all of those allowable? No, we need to eliminate any that don't have at least two heads.
How many ways are there to get 0 heads? 1
How many ways are there to get 1 heads? 6
Okay, so that's 7 configurations that aren't possible because we know we have to have at least two heads.
That leaves us with 64-7 = 57 allowable configurations.
How many of them "win?" Just one: HHHHHH.
So, the probability is 1/57.
Answer choice D.
For those who are curious, let's address why 1/2^4 doesn't work.
That would be the answer if we knew that two SPECIFIC coins landed heads. Imagine I say that first two coins land heads and ask what's probability that the remaining four also land heads. We'd have H H _ _ _ _. Now your 1/2^4 would work. But we just know that SOME two coins landed heads.
ThatDudeKnowsProbability
General Discussion
09 Feb 2021, 18:13
Kudos
Bookmarks
doomedcat
What is the probability that all 6 coins tossed together will turn up heads, if it is known that 2 of them have turned up Heads?
A) 1/4
B) 1/16
C) 1/64
D) 1/57
E) 7/57
Solution: A) 1/4
B) 1/16
C) 1/64
D) 1/57
E) 7/57
Since we know that 2 of the 6 coins will turn up heads, the probability that the other 4 coins will also turn up heads is ½ x ½ x ½ x ½ = 1/16.
Answer: B
