BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

★★★★★5.0559 reviews
GMATLiveTeach 7 seats left
Chris Peckover
NEXT LIVE COHORT

Oct 13 to Jan 7, 2027

with Chris Peckover

Schedule
Tue, Thu · 8:00 to 10:00 PM ET
Included
40 live hours + 6 months of GMAT OnDemand
  • Live instruction and real-time questions
  • Class recordings and assigned practice
View class & enroll
Limited cohort · enrollment openTarget Test Prep
EALiveTeach 5 seats left
Logan Thompson
EXECUTIVE ASSESSMENT

Sep 6 to Dec 6, 2026

with Logan Thompson

Schedule
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll
Limited cohort · enrollment openTarget Test Prep
GMATOnDemand Start anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 559 reviewsTarget Test Prep

Compare the format, schedule, and included access before enrolling. Prices and seat counts shown reflect the supplied offer details.

Probablity - A certain telephone number has 7 digits.

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Problem Solving |
Senior | Next Rank: 100 Posts
Posts: 59
Joined: Mon Mar 10, 2008 3:23 amFollowed by: 1 member
Thanked: 2 times

by Magellan » Thu May 15, 2008 3:57 am
We have 4 digits (7 minus 3 fixed) that can be potentially prime.

Those digit can take the following values: 2, 3, 4, 5, 6, 7, 8, 9
Out of those digits, half are prime: 2, 3, 5, 7

To get to the result, we will use:
Probability(At least 1 prime) = 1 - Probability(No Prime at all)

We know:
Probability no prime for 1st digit: 1/2
Probability no prime for 2nd digit: 1/2
Probability no prime for 3rd digit: 1/2
Probability no prime for 4th digit: 1/2

Therefore, probability(No prime at all) = 1/2 * 1/2 * 1/2 * 1/2 = 1/16

And we get
Probability(At least 1 prime) = 1 - Probability(No Prime at all) = 1 - 1/16 = 15/16
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 73
Joined: Fri Feb 01, 2008 1:28 amFollowed by: 1 member
Location: India
Thanked: 1 times

by sandeep_chhabra » Thu May 15, 2008 4:03 am
Oops !! it was my bad :(

i did the same but finally forgot that i have got the probability of No Primes and need to subtract it from 1.

Thanks!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1