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We've got a scale here, and as you see,

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the scale is balanced.

3
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And we have a question to answer.

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We have this mystery mass over here.

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It's a big question mark on this blue mass.

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And we also have a bunch of 1 kilogram masses.

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These are all each a 1kg mass.

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And my question to you is:

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What could we do to either side of this scale

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in order to figure out

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what the mystery mass is?

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Or maybe we can't figure it out at all?

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Is there something that we can do

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either removing or adding these things,

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so that we can figure out

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what this mystery mass is?

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I will give you a couple of seconds to think about it.

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To figure out what this mystery mass is,

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we essentially just want this on one side of this scale

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But that by itself is not enough.

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We could just remove these three,

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but that won't do the job, because

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if we just remove these three,

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then the left side of this scale

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is clearly going to have less mass,

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and it will go up, and the right side will go down.

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And that will not give us much information.

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It would just tell us that this blue thing

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has a lower mass than what is over here.

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So just removing this will not help us much.

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I won't let us know that this is equal to that.

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What we have to do if we want to keep the scale balanced,

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is that we have to remove the same amount of mass

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from both sides of the scale.

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So, if we want to remove 3 things here,

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(let me try my best to remove 3 things here)

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(I will just erase it)

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If we want to remove 3 things there,

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If we did this by itself,

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just removed these 3 things,

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the two sides would not have an equal mass anymore.

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this side would have a lower mass.

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So we have to remove 3 from both sides.

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If we want to make sure our scale is balanced,

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we have to remove 3 from both sides.

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If we started off with the scales balanced,

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and then we removed 3 from both sides,

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the scale will still be balanced

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and then we would have a clear idea

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of what the mass of this object actually is.

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Now, with 3 removed from both sides,

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the scale will still be balanced,

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and we know that this mass

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is equal to whatever is left over here.

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It is equal to 1, 2, 3, 4, 5, 6, 7

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- and if we're assuming they are kilograms -

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we'll know that the question mark mass

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question mark is equal to 7 kilograms.

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So this is a seven kilogram mass.