I'm a computer programmer and I'm working about this problem. I must say that I'm not very familiar with British English and I'd like to know when the word and is used in the numbers. The perfect answer for me will be something like "The word and is used with numbers not divisible by 10" (something mathematical). But all answers will be appreciated.
For British English, numbers greater than 100 and less than 1000 always include and between the “hundreds” figure and the other part:
Three hundred and forty-two
Numbers greater than 1000 include and between any “hundreds” figure and the figure less than 100, and then state the multiple:
Five thousand, three hundred
Three hundred and forty-two thousand, two hundred and fifty-one
One million, three hundred thousand, four hundred and twenty
Two thousand, three hundred and sixty-two million
That is, any of the three-figure groups in a large number follow the first rule before stating the multiplier.