Probability Equation Questions

What are Probability Equation Questions?

Probability Equation Questions involve forming and solving an equation to find the missing probability. They often involve forming a quadratic equation so you need to be confident with conditional probabilty as well as solving quadratic equations before attempting these questions.

Probability Equation Questions Example Question (Edexcel Higher November 2018, Paper 1, Question 22)

Example Question

There are only green pens and blue pens in a box.

There are three more blue pens than green pens in the box.

There are more than 12 pens in the box.

Simon is going to take at random two pens from the box.

The probability that Simon will take two pens of the same colour is 27/55

Work out the number of green pens in the box.