A maths question

Solution :
let x be the money of mary had at first
from the question , we get
( 1 - 0 .4) x = 80
0.6 x = 80
x =$ 400 / 3
Mary has $400/3 (wrong)

We don't fraction in our money, if it can become a decimal, it's ok but the answer can change the unit to decimal. This is a irrational number(無理數), So we divide it first, 400/3 is equal to 133.33333......, and we do it in a way like this:$133.3333.......become $134.

Your friend say it is not fraction, it is because of this reason that we don't have any money with a irrational number.

i think i understand what is the argument between you and your friend

suppose i have 6 ppl here, and one taxi can only contains 5 ppl
so i need 1.2 taxi for this 6 people, but no one can pick a 0.2 taxi or 0.5 taxi orsomething else , it must have 2 taxi , i think what you friend is trying to say is something like that'

and 80x 5/ 3 = 400 / 3 , i think it is the best answer you can have

let mary had $Xmoney before.

(1-0.4)X=80
0.6X=80
X=133.3333333333333333333333333333333333333...