Simple Maths Question

[skiba]

Apparently this is a 12 year-old math's question.

I suck at maths, do you?

820 boys and girls in total at a fair
3/4 boys and 3/5 girls left
120 more girls that boys remained
what is the initial number of boys

 

[marc2003]

ok, so i failed miserably at cheating but how the hell does google have this 4 minutes after you posted.

 

[skiba]

I can't see the pic, what is it?

 

[marc2003]

this thread is the first result on google for "820 boys and girls in total at a fair" 4 minutes after it was posted.

http://www.google.com/search?q=820+...s=org.mozilla:en-GB:official&client=firefox-a

 

[skiba]

Not sure if i'm impressed at their efficiency or scared...

 

[marc_howarth]

320 boys at the start

It's a simple simultaneous equation.

lets say the number of boys at the start is x
and the number of girls at the start is y

"820 boys and girls in total at a fair" becomes
x + y = 820


"3/4 boys and 3/5 girls left
120 more girls than boys remained" is slightly trickier

if 3/4 boys left, it means there are 1/4 of the boys remaining.
similarly if 3/5 girls left, there are 2/5 girls remaining

now we know there are 120 more girls so it is possible to say
1/4*x + 120 = 2/5*y

this can be re-arranged to:
-1/4*x + 2/5*y = 120

which can be solves similtaneuosly with x + y = 820

 

[Haircut]

OK, if we say:
b = total number of boys
g = total number of girls

(g - (3/5)g) - (b - (3/4)b) = 120
b + g = 820, so g = 820 - b

And it boils down to a simple simultaneous equation

(820 - b) - (3/5)(820 - b) - b + (3/4)b = 120

820 - b - 492 + (3/5)b - b + (3/4)b = 120

328 - (13/20)b = 120

(13/20)b = 208

b = 320

 

[skiba]

easy as that....thanks mate.

 

[Razor-BladE]

It's a trick question, why would women be at a fair when they're in the kitchen?

 

[D.Roberts]

It's a trick question, why would women be at a fair when they're in the kitchen?

+2 nice

 

[Artheas]

b + g = 820
(1/4)b + 120 = (2/5)g
5b + 2400 = 8g

b = 820 - g
4100 - 5g + 2400 = 8g
6500 = 13g
g = 500
b = 320