American vs. British English: meaning of "One hundred and fifty"

When I learned this “rule” (in first grade, I believe), it was explained that and separates the whole part from the fractional part: 2⅔=two and two thirds. The word and would only represent the decimal point in decimal numbers when they are read out in the formal “fractional” reading of decimals, as 2.3=two and three tenths, or 1.75=one and seventy-five one-hundredths. That is, according to this rule, *one hundred and fifty is ungrammatical because, if it is supposed to mean 150, it should be one hundred fifty, and if it is supposed to be mean 100.50, it should be one hundred and fifty one-hundredths. The rationale behind the rule is that you should only have one and in a phrase, so if the number were 403⅞, you wouldn’t say four hundred and three and seven eighths.

Of course, most of the time the decimal point is read as point: 2.3=two point three; 1.75=one point seven five or one point seventy-five; 100.50=one hundred point five zero, one hundred point five oh, or one hundred point fifty. The fractional reading of decimal numbers also starts to become a bit ridiculous if there are more than three digits after the decimal point: nobody would say 3.14159265=three and fourteen million one hundred fifty-nine thousand two hundred sixty-five one hundred millionths.

As you have undoubtedly observed, many Americans don’t follow the rule about and only being used to separate whole and fractional parts, and insert and just before the units of a number less than one hundred, although the forms without and are quite common too.

457 four hundred fifty-seven or four hundred and fifty-seven
2001 two thousand one or two thousand and one
1,000,001 one million one or one million and one