Forty
numeral
What do you get when you have four tens? You get forty.
よんじゅう
This is a jukugo word, which usually means on'yomi readings from the kanji. The only exception to this is 四, which uses the kun'yomi reading, since the on'yomi reading sounds too much like the word for "death." Luckily you know these reading already, so there's nothing new to learn!
Tシャツを四十まいもっています。
I have forty T-shirts.
あの天才アメリカ人は四十才です。
That genius American is 40 years old.
四十だいにもなると、ぜんしんがかゆい。
Since reaching my forties, my entire body has been itchy.