Why is it solved like this

[Roflmao]

Question 1: In an examination 40% students failed in Math, 30% in English and 10% in both. Find the % that passed in both?

Answer: required %= 100-(40-30-10)= 40%

If i did it as 40+30+10=80 & 100-80=20% then the answer is wrong. How do they get 40%?


Question 2: 40 liters of a mixture of milk and water contains 10% of water. How much water should be added so that water is 20%?

The answer according to me: Since water is 10% already we need 10% more to make it 20%, so 10% of 40= 4 liters

BUT the answer is 5 liters! WHY??? EXPLAIN PLEASE

[Collin Li]

Question 1: In an examination 40% students failed in Math, 30% in English and 10% in both. Find the % that passed in both?

Answer: required %= 100-(40-30-10)= 40%

If i did it as 40+30+10=80 & 100-80=20% then the answer is wrong. How do they get 40%?

You should be doing 40 + 30 - 10 = 60, then 100 - 60 = 40.

The reason why we subtract 10 (both) is because they were double-counted in both the Math and English percentages.

[Collin Li]

Question 2: 40 liters of a mixture of milk and water contains 10% of water. How much water should be added so that water is 20%?

The answer according to me: Since water is 10% already we need 10% more to make it 20%, so 10% of 40= 4 liters

BUT the answer is 5 liters! WHY??? EXPLAIN PLEASE

The reason why that doesn't work is because when you add water, you increase the volume above 40, so it is no longer 20% of 40L anymore.

[Roflmao]

oh

[Roflmao]

10% of the mixture is water meaning that water comprises 4 L of the 40 L mixture. They're asking you for the number of liters to be added so that the mixture has 20% water. So, to express this in algebraic terms:

4 + x   20
40 + x =  100

Solving for x yields the number of liters of water to be added.

I get it i think

?

[Collin Li]

Do you mean solving for:

Yeah. That would be right. The steps following would be:

[Roflmao]

Oh lol im not smart everyone knows more than me xD

[costargh]

well you're only 6 years old... it's to be expected

[Collin Li]

Hm... algebra in grade 4 is pretty extreme!

* coblin pats you on the back

[Roflmao]

Im not 6 I take that as a insult

[bucket]

this is like, year8/9 maths, doing it in grade 4 is beyond extreme :S

[Roflmao]

lol im smart

[Flaming_Arrow]

The Next Ahmad?

[Roflmao]

Maybe... you choose

[Flaming_Arrow]

i don't choose you