noun, plural thir·ties. a cardinal number, 10 times 3. a symbol for this number, as 30 or XXX.

Is it 40's or forties?

40 (forty) is the number that follows 39 and precedes 41. Though it’s related to the number “four” (4), the modern spelling of 40 is “forty.” The older form, “fourty,” is treated as a misspelling today.

What's the meaning of thirties?

When you talk about the thirties, you are referring to numbers between 30 and 39. For example, if you are in your thirties, you are aged between 30 and 39. If the temperature is in the thirties, the temperature is between 30 and 39 degrees.