Arthur has $90 and spent $40 and gave the $20 to his brother. The total cost of the money he gave was $60. It leaves him with only $30 to keep. If you'll do the math, Arthur had already spent ⅔ of his money. Let’s say that $90 is equals to a whole number 1 or 3/3 and $60 was its ⅔. Subtract ⅔ ($60) from 3/3 ($90) which will leave you with ⅓ . Only ⅓ of Arthur's money was left after spending it.