Explanation:
sum of numbers divided by how many numbers there are.
(2/4 + 18/5 + 28/10 + 5/20)/4
we bring all fractions to .../20, as 20 is the smallest common multiple of all denominators. and then we can easily sum them all up.
so,
(10/20 + 72/20 + 56/20 + 5/20)/4 =
143/20 / 4 = 143/20 × 1/4 =
= 143/80 is the mean or average