Question

There is a bag with only red
and blue marbles. The probability
of choosing a blue marble at
random is 2/7.Given that there
are 84 marbles in total, and each
one is likely to be chosen at
random work out out how many
red marbles there must be.

Answer 1

Answer:

Step-by-step explanation:

This can be solved with a very basic knowledge of probability and proportions. If we have 2 chances out of 7 to pick a blue marble, then we have 5 chances out of 7 to pick a red one, right? We can use that and set up a proportion with red marbles on top and total number of marbles on the bottom:

$\frac{red}{total}:\frac{5}{7}=\frac{x}{84}$

5/7 is the reduced fraction that represents the number of red marbles over the total number of marbles. Solving this way allows us to find out what the fraction was before it was reduced, hence, giving us the total number of red marbles. Cross multiply to solve:

7x = 420 so

x = 60. 60 red marbles total