Question

A jar contains 8 white marbles, 10 orange marbles, and some red marbles. If the probability of picking a red marble is 1 in 4, then how many red marbles are there?
[4 MARKS]

Answer 1

Formula: 1 Mark
Calculations: 2 Marks
Answer: 1 Mark

Let the number of red marbles be x.
Total number of marbles = 8+10+x = 18+x

Given, the probability of picking a red marble is 1 in 4.
$\Rightarrow P\left(pickingredmarbles\right)=\frac{x}{(8+10+x)}=\frac{1}{4}$
$\Rightarrow 4x=8+10+x\Rightarrow 3x=18\Rightarrow x=6$
Hence, there are 6 red marbles.