# Probability or Algebra ?

Discussion in 'Computer Support' started by Angela Singh, Sep 13, 2006.

1. ### Angela SinghGuest

Hello,

I need two lots of help - firstly is this Algebra or Probability,
and secondly - what is the answer ?

I made a start (below) but have reached my limit

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Two pupils were to be chosen at random from a school register to take
part in a competition. The probability that both would be boys was
one- third. Before the choice could be made however, a decision was
taken to include pupils from the register of another school in the
ballot for the two places. This other chool had a register of 1,000
pupils, and the chance that the two selected pupils would both be boys
was reduced to one-thirteenth.

How many pupils are on the register of the first school ?
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So far i have worked out that if the chance of two boys from school A
is one third, then the chance of the first picked being a boy is
1/sqrt*3 which is approx 57.7%
Therefore school A's breakdown is 57.7% male and 42.2% female

Population od school B = 1000

The chance of 2 boys from (school A + 1000) = 1/13

therefore the chance of 2 boys = (A + 1000) = 27.7%

therefore the breakdown of (A+1000) = 27.7% male and 72.3% female

Many thanks,

Angela

Angela Singh, Sep 13, 2006

2. ### richardGuest

First, read what the question wants as an answer. That being, the population
of school A.

What number gives 2 as 1/3 of that number?
6.

The rest of the problem is there only to throw you off.

Like in the age old riddle, "There once was a man who was going to Tipperary
by train. At the train station he meets a woman with 7 kids who was going to
London. Each of the kids had 7 goats.............How many were going to
Tipperary?"

You have two coins totalling 55 cents. One is not a half dollar coin. What
is the other?

richard, Sep 13, 2006

3. ### Angela SinghGuest

Thank you Richard, but - If school A has 6 pupils, then
the chance of the first selected being a boy is 3 out of 6 (= 1/2)
and then there are only 5 pupils left in the population because one
has already been selected and removed.

The probability that the first two selected are boys = 1/3

therefore, chance of first two being being boys is (1/2) * (X) = 1/3

therefore the chance of the second selected being a boy (X) = 2/3

And what about the chance of the first two selected pupils being boys
from school A + school B (6+1000) being one-thirteenth ?

I have to submit this answer by tomorrow but i want to understand it
first.

Thanks,
Angela

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Two pupils were to be chosen at random from a school register to take
part in a competition. The probability that both would be boys was
one- third. Before the choice could be made however, a decision was
taken to include pupils from the register of another school in the
ballot for the two places. This other chool had a register of 1,000
pupils, and the chance that the two selected pupils would both be boys
was reduced to one-thirteenth.

How many pupils are on the register of the first school ?

Angela Singh, Sep 14, 2006