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!
四十才になった日、ニューヨークにいました。
I was in New York on the day I turned forty.
このテーブルクロス、四十センチに切って下さい。
Cut this tablecloth to 40 cm, please.
人生で四十台ものスマホをかってきた。
I've had forty smartphones in my life.