[Closed] Is 100 million a tenth of a billion?

Yes.

A billion is a thousand million, in the UK and the US these days. The old UK 'million million' is outdated.

Here you go,

https://en.wikipedia.org/wiki/Long_and_short_scales

Many countries, including most in continental Europe and Latin America, use the long scale whereas most English-speaking countries and Arabic-speaking countries use the short scale.
...
For most of the 19th century and into the early 20th century, the United Kingdom largely used the long scale, whereas the United States of America used the short scale, so that the two systems were often referred to as British and American in the English language. After several decades of increasing British usage of the short scale, in 1974, the government of the UK fully adopted it, which is reflected in its mass media and official usage. With very few exceptions,the British usage and American usage are now identical.

I believe that to be so. We fell in line with the US I believe.

I am sure that when I was at school a Billion was a million million but the US like to use impressive words more often* so a Billion got downgraded.

*See 'weather bomb' as a term used by the US to mean 'quite a large storm'.