WAEC 2020 · Paper 1 · Q34

A bag contains 5 red and 5 blue identical balls. Three balls are selected at random without replacement. Determine the probability of selecting balls alternating in colour.

Worked solution (try it first)
  1. Alternating colours means red, blue, red or blue, red, blue.
  2. P(RBR)=510×59×48P(\text{RBR}) = \frac{5}{10} \times \frac{5}{9} \times \frac{4}{8}
    =100720= \frac{100}{720}
    =536= \frac{5}{36}.
  3. By symmetry P(BRB)=536P(\text{BRB}) = \frac{5}{36} too.
  4. Add the two: 536+536=1036\frac{5}{36} + \frac{5}{36} = \frac{10}{36}
    =518= \frac{5}{18}, option B.

Report a problem with this question