5lb Book: p 680, # 6

[bilon.geiger]

I'm having trouble solving this problem by picking numbers. I understand and am comfortable with the algebraic approach, but when I pick numbers, I don't consistently get the right answer.

For example, if I choose X=2 and T = 10, I get the right answer. However, if I choose X=2 and T=16, I get the wrong answer. I can't figure out what the issue is so that I know to look out for this moving forward.

Thanks for your help.

[tommywallach]

Hey Bilon,

Try talking through the math on here. I promise you, plugging in won't stop working with a different set of numbers. All that's happening here is you're doing the arithmetic wrong. I promise!

-t

[bilon.geiger]

Hi Tommy,

You're right - I'm doing something wrong, but can't figure out what.
If T = 10, X = 2, then here is my math for T & K's age in x years:

(T+2) = 3(K+2)
(10+2) = 3(K+2)
(12) = 3K+6
12-6 = 3K
6=3K
2=K

I don't think this is right because if I look at the answer choices and plug X=2, then I end up with 2X = 4, but I just calculated K's age as 2. I'm confusing myself at this point.

Thanks for your help.

[tommywallach]

Hey Bilon,

Hey there. This is my fault, because I didn't check out the question first. Remember your process for plugging in (on a VIC: Variables in Answer Choice).

1. Plug in for variables used in answers
2. Solve to answer
3. Plug back into answer choices

Your mistake here was setting an age value. You can only plug in for x, because it's the only variable that shows up in the answer choices.

x = 4 (just made that up)

t = 4k
t + 4 = 3(k + 4)

Now we solve by substituting:

4k + 4 = 3k + 12
k = 8

Now we can see that Kosuke is 2x. (2*4 = 8)

Make sense?

-t

[bilon.geiger]

Yes Tommy, that makes sense. Thank you!

[tommywallach]

Glad to help!

-t