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### Course: Algebra basics>Unit 3

Lesson 7: Writing & solving proportions

# Solving tricky proportions

Sal solves the equation (x-9)/12=2/3. Created by Sal Khan.

## Want to join the conversation?

• at , why did Sal add 7 to the 7x? (7x + 7)
• Distributive property. 7(x + 1) = 7(x) + 7(1).
Therefore, 7(x + 1) = 7x + 7.
• oh and for some people, at when he says what x equals to. You have to divide 51 by 3. Just saying in case someone gets confused
• The number of sparrows sitting on the top of the fence is inversely proportional to the square of the velocity of the wind. At noon there were 18 sparrows on the fence with the wind speed at 6 mph. At 2pm, there are 6 sparrows on the fence. What is the wind speed in mph rounded to the nearest tenth? (I have the answer--10.4 mph. How do you work it?)
• first you need to set equation for number of sparrows (N) in relation to velocity (v). they are inversely proportional so we can write N=C/(v^2) ; C-constant. Constant is needed because we don't know anything about relations except that they are inversely proportional.
Then we can calculate constant from first condition: N=18 and v=6mph
And finally, we can calculate wind speed.
• What do you mean "hairy proportions"?
• He never mentions hairy proportions.
(1 vote)
• Why do they even call this HAIRY proportions??
• He calls it hairy because it can be tricky if you don't proceed correctly.

Hope this helps!
• Thank you Sal it always help when I miss a day in algebra and I have no clue what im doing. My brother told me he uses you and when I first tried I saw its the same thing my old math teacher used to help teach us. It also helps since im in a high school course but im in middle school. Thanks again Sal your a big help!!
• How would I solve a proportion with variables in both denominators?
(1 vote)
• Same way: cross multiply.
Say we had 2/x+1 = 3/3x-2.
If we cross multiply we get 2(3x+2)=3(x+1)
Using the distributive property...
6x+4=3x+3
Then, solve like a normal equation. subtract 4 from both sides to get
6x=3x-1
Subtract 3x from both sides to get
3x=-1
And then divide both sides by 3 to get
x=-1/3
Hope this helps!
• 51 into 17 how did you do it could you possibly explain I think I might have to pause the video and put it back at that part huh?
(1 vote)
• Good question. At , Sal writes:

`3x = 51`

The next step would be:

`x = 51/3`

And finally since 17 times 3 is equal to 51:

`x = 17`

Hope this helped. Here's a great tip for you - next time you ask something specific with regard to the video, use a timestamp by mentioning the time like I did above referring to the point in the video at 48 seconds.