Math question

[far123]

Hey guys, this question seems so easy, but I can't get d answer!

The age if a father and his step son is 3:2 . four years before , the ratio was 5:4, what is the present age of the father?
a) 12
b) 24
c) 36

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[Mstoothlady2012]

Hey guys, this question seems so easy, but I can't get d answer!

The age if a father and his step son is 3:2 . four years before , the ratio was 5:4, what is the present age of the father?
a) 12
b) 24
c) 36

For this kind of problems, I would just do backsolving. I would start with B.

The father's age is 24 NOW. Therefore the son must be 16 (b/c the ratio is 3:2)
So before 4 years father was 20 (24-4) & son was 12 (16-4). Now this ratio should be equal to 5:4, which it is. So the answer is B.

 

[drgreen]

let f=fathers age, and s=sons age

we know
f/s=3/2
f=3s/2

we also know that
(f-4)/(s-4)=5/4
4(f-4)=5(s-4)
4f-16=5s-20
4f=5s-4
f=(5s-4)/4

Plug in first equation for f
3s/2=(5s-4)/4
6s/4=(5s-4)/4
6s=5s-4
s=-4

plug into f=3s/2
f=3(-4)/2
f=-6

The father's age is -6

Theres a prob with the question

 

[drgreen]

Mstoothlady2012 would be right if the question had asked a ratio of 5:3 not 5:4

 

[Mstoothlady2012]

Mstoothlady2012 would be right if the question had asked a ratio of 5:3 not 5:4

you're right! I miscalculated the ratio at the end.

 

[Streetwolf]

When you subtract the same number from numerator and denominator, the fraction you end up with should be GREATER than the original fraction (you need both to be positive though). In this case we go from 3/2 to 5/4 and obviously 3/2 > 5/4. You need it to be 5/3 to have a correct answer.

 

[pistolpete007]

so he became a father at the age of 8? lol

 

[Streetwolf]

so he became a father at the age of 8? lol

Well it DOES say step son. Still kinda weird though.

 

[far123]

thanks guys, I think Streetwolf is right...cause, in Crack Math 12 is the answer. but, it does not make sense!!
3y=2x
5(y-4)=4(x-4)
4(x-4)=10x/3-20
???? -x/3=-20+16
x=12

 

[harrygt]

NOne of the answers are correct.
NEXT!