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### Course: Pre-algebra>Unit 7

Lesson 9: Analyzing relationships between variables

# Writing equations for relationships between quantities

In this lesson, we learn how to write equations representing real-world scenarios. First, we explore how to calculate time taken for a walk based on speed and distance. Then, we discover how to determine the amount of water needed in a soup recipe based on the quantity of vegetable stock. Fun, right? Created by Sal Khan.

## Want to join the conversation?

• hi how can two variables relate to each other pls make it easy
• Hi "pls sub to adrain not pro g..."
Fundamentally, it means that the values of variable correspond to the values of another variable, for each case in the dataset. In other words, knowing the value of one variable, for a given case, helps you to predict the value of the other one
-Relationships Among Variables (analytictech)
• Hi, I have a question in the exercises (2.1):
I think it should be w=5/4*v, because 4w=5v (400 ml of water for every 500 ml of vegetables), and we dived both sides with 4, and the result is w=5v/4, that is the same thing as w=5/4*v
• actually the question is indirectly wrong ,as the question is wrong it may can be 4/5 or 5/4 so in any way it can be correct
• I don’t understand how u divide a number by a fraction like he did in the video. Can someone explain it for me?
(1 vote)
• You multiply it.
• if there wasn't 6 in the first question how will we do
• On the 2nd question from the last problem, how did he get 4/5? The decimal equivalent is 0.8 and it doesn't add up anything in the table he did. 400 x 0.8 = 320, not 500. 400 x 1.25 = 500. 800 x 1.25 = 1000. 4 x 1.25 = 5. So I'd make sense that the equation would be w = 1.25v, right?
(1 vote)
• The table was a ratio, if we take off the extra zeros, we get for every 4 units of water, we use 5 units of vegetable stock. So, the equation does not have anything to do with decimals and instead uses 4/5. This is how we get w= (4/5)v On the second problem we get a value for v, so we do (4/5) x 800. Let me know if I did anything wrong.
(1 vote)
• Can someone help me and explain it better please. I can't understand it.
• Hmm... how does multyplying 5km/h times 2hrs give us 10km? I don't see how those units (km/h, hrs) give us "km" in the end.
• Try to look just for the units, then you'll have km/h * h, so we can rewrite this like (km * h)/h.
Notice that h/h is equal to 1, so they "cancel" each other, just the km stays there. Looking this way, the final unit must cleary be the km.