You are here:

Advanced Math/Probabilities

Advertisement

Expert: Robi Bhattacharjee - 3/4/2009


Question
list consists of p positive and n negative numbers. if a number is picked randomly,the probability the number is positive is 3/5. what is the value of n/p?
you answered this for a friend, but i cant seem to understand how u got to the step 3p + 3n = 5p?  Help!

Answer
Hi William,

To solve this problem, a systematic approach is your best bet.

You know there is a total of p + n numbers.

You also know that there is p positive numbers.

So the probability of getting a positive is p/(p+n) which we know = 3/5

So p/(p+n) = 3/5 if we take reciprocals of both sides

(p+n)/p = 5/3  now we expand

p/p + n/p = 5/3

1 + n/p = 5/3

n/p = 5/3-1 = 2/3.

As for your friend, he probably got this from cross multiplying when you have

p/(p+n) = 3/5

5p = 3(p+n)

5p = 3p + 3n

However, obtaining this isn't necessary to solve the problem as I showed.

I hope this helps,
Robi

Advanced Math

All Answers

Answers by Expert:

Ask Experts

Volunteer

Robi Bhattacharjee

Expertise

I can answer a variety of questions on mathematics. Questions on trigonometry, calculus(preferably single variable), algebra, geometry, and number theory will be answered. I cannot answer questions on abstract branches of mathematics such as group theory. I also cannot answer questions on statistics. In number theory, I can answer questions on congruences, prime numbers, units, functions, and the riemann-zeta function.

Experience

I have studied advanced math my entire life. I started calculus in sixth grade. I have attended numerous math competitions and I am attending math organizations such as the San-Diego math circle. Also, this year I have been invited to the USAMO which is a prestigious math competition (Every year the USAMO invites 500 students from across the USA to participate in this competition. The top 6 go to represent the USA in the International Math Olympiad).

Organizations
I am in the San Diego Math Circle

Education/Credentials
I am entering high school and have received a perfect score and the STAR test 5 times in a row. I also have gotten recognitions in the AMC 10, AIME, Math Counts, and ARML. Additionally, I have won the San Diego Math Olimpiad twice in a row.

©2012 About.com, a part of The New York Times Company. All rights reserved.