# Substitution Problem

1. Dec 30, 2009

### henrbp29

1. The problem statement, all variables and given/known data

d1/d2 + d2/v2 = 0.500km/v1 + 3.50km/2v1 = 0.250h

2. Relevant equations

v2 = 2v1

3. The attempt at a solution

The answer is 9.00km/h but I keep coming up with 5.33. I know this is simple algebra but I am missing something in the order of operations.

0.500km/v1 + 3.50km/2v1 = 0.250h

4.00km/3v1 = 0.250h

4.00/(0.250h)(3) = v1

v1 = 5.33 ??
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 30, 2009

### RoyalCat

$$\frac{a}{b}+\frac{c}{d}\neq \frac{a+c}{b+d}$$

At this point:
$$\frac{0.5 km}{v_1}+\frac{3.5 km}{2v_1}=0.25 hr$$

Multiply both sides by $$v_1$$ and remember how to add fractions!

$$\frac{a}{b}+\frac{c}{d}= \frac{ad+bc}{bd}$$

3. Jan 2, 2010

### henrbp29

Thanks again for your help!! I'm new to the forum and still learning to navigate the site. I am taking an online physics course and will be needing your help in the future I'm sure. I'll be in touch.

4. Jan 2, 2010

### RoyalCat

Sure thing man. :) But I suggest you work a bit on your algebra first, since you'll be using it a lot!