There are n n\,n pens in a box. 14 of the pens are black. 6 of the pens are green. The rest of the pens are red. One of the pens is chosen at random. The probability that the pen is red is 57\displaystyle \frac{5}{7}75.
Find the value of nnn.
236 exam-style questions on WJEC GCSE Maths Probability. Each one has a worked solution and a mark scheme showing where the marks go.