Math Is Fun Forum

Ubergeek

Member

Registered: 2009-07-14

Posts: 205

A few exs for test-preparation

1) A number times two, added twelve and then the result divided by two equals fifteen. What's the number?

2) Find two positive consecutive integers whose squares' sum equals 481.

3) A student gets 5 points for every exercise he gets right, and looses 3 points for each he gets wrong. After doing 50 exercises he had 130 points. How many did he got right?

4) The sum of two brothers ages equals 30 years now. 8 years ago, the product of the ages was 48. What's the older brother's nowadays age?

Enjoy.

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"Mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true."

Bertrand Russell

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: A few exs for test-preparation

Last edited by bobbym (2009-07-27 07:27:08)

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

rzaidan

Member

Registered: 2009-08-13

Posts: 59

Re: A few exs for test-preparation

Dear Ubergeek

for no. 3:
Assume that the no. of right exercises = x and no. of wrong exercises = y
Therefore
x+y=50       ⇒          y=50-x
and
5x-3y=130 ......(1) substitute  y=50-x  in  (1) you get   5x-3(50-x)=130
or
5x-150+3x=130

8x=280    x= 35   ,    y=  15
so the no. of right exercises = 35

the no. of wrong exercises = 15

For no. (4)

Assume that the age of the first = x  now
                   the age of the second= y  now
therefore x+y=30          y=30 - x
before 8 years (x-8)(y-8)=48..........(1)  but  y=30 - x

(x-8)(30-x-8)=48  so (x-8)(22-x)=48 

22x-x^2-176+8x=48  ⇒  x^2-30x+224=0  ⇒  (x-16)(x-14)=0
x=16     and    y=14 or

x=14   and     y=16
So the older brother's nowadays age = 16
Best Wishes
Riad Zaidan