Number 28 on postage stamp

28

= 2 × 2 × 7

= 2 × 14
= 4 × 7

= 1 + 2 + 3 + 4 + 5 + 6 + 7
A triangular number.

There are 28 days in February, except in leap years.

28 is a perfect number because it equals the sum of all its factors (apart from 28 itself).

An out-stretched arm and fingers illustrating cubits and digits.

There are 28 digits in one cubit. The ancient Egyptians had their own system for measuring length based on body lengths. A digit was based on the width of one finger (about 19 mm). A cubit was the distance measured from the elbow to the tips of the fingers, and this equalled 28 digits.

Use a ruler to measure a digit and a cubit on yourself. How many of your digits make one cubit? How close is it to 28? Take measurements from a number of different people. How close are they to 28?

Number 28