The first thing to note about the expression of numbers is that there is a difference between the way certain numbers are expressed in everyday speech and mathematics.
Most of us take the mathematical manner of expressing numbers most probably because we were first exposed to numbers in mathematics classes more than in any other field.
For instance the figure 0 can be called ‘o (as an alphabet), nought/ naught, nil or zero’ but most people are used to expressing it as ‘zero’ wherever the figure is found.
Figure 0 should be called ‘zero’ in mathematics and sciences where it essentially means ‘nothing in value’.
You are right to express the temperature of a certain area as ‘freezing below zero’ but it is wrong to say ‘zero’ when expressing a telephone number in speech. You rather call it ‘oh or nought/ naught’.
Take an example of the figures for the phone number of Rwanda Revenue Authority desk: +250 252595500. We say it as: ‘plus two five o, two five two five nine double five double o’.
Note that telephone numbers are not combined when expressing them in speech. They are individually expressed as is above. Do not say ‘plus twenty-five …’
You will also hear some people using the word ‘nought/ naught’ for figure 0. This is purely British English.
When mentioning the scores of teams in games, for example, teams A-B scored 1-0 we express it in speech as ‘team A scored one and B scored nil’ or ‘one nil’. Avoid saying ‘zero’ or ‘o’ in this aspect.
When writing a single digit number, always write it in words, especially in an assay. You can report that ‘25 people were at the meeting but by the end, nine more had joined’ not ‘9 more had joined’.
Also remember to add a hyphen in words expressing numbers with double digits for instance 25 as twenty-five, 25th as twenty-fifth.
When counting a form of money, always keep the name of the number singular. We say ‘10 million not millions. The reason is money is uncountable. However we can say ‘millions of francs’.