11 slides, with what the presenter says. Highlighted words are cues: saying them makes something on the slide move.
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1
Cover
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Today we are adding fractions, and we are using pizza and chocolate to do it. Here is the funny thing about fractions. Halves and thirds don't mix, at least not yet. By the end of class you will know how to make them mix.
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2
Two pizzas
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Here is a half of one pizza. And here is a third of another pizza. If I eat both pieces, how much pizza did I eat? Take a guess and whisper it to your neighbour. Now look closely, because those two pieces are not the same size.
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3
The wrong answer
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A lot of people add the tops and add the bottoms, and they get two fifths. That answer is wrong. Look at the bar. Two fifths is less than one half, but we ate more than half a pizza! We can only count pieces together when they are the same size.
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4
Common denominator
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We need pieces that are the same size, so we need one number both bottoms fit into. Let's count by twos. Two, four, six, eight. Now count by threes. Three, six, nine. Look for the first match. It's six, so we cut everything into sixths.
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5
Convert to sixths
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Now we cut both pizzas into sixths. The amount of pizza stays exactly the same. Cut the half again and it is three sixths. We multiplied the top and the bottom by three. Cut the third again and it is two sixths, times two on top and bottom.
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6
Add the tops
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Now every piece is a sixth, so we can count them together. We add the tops. Three plus two is five. The bottom stays six, because the slices did not change size. That's five slices out of six.
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7
Simplify
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Our last step is to make the answer as simple as we can. Say we got four sixths. We divide the top and the bottom by two, and we get two thirds. Same amount of pizza, smaller numbers. But not every answer shrinks, so always check. Five sixths is already as simple as it gets.
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8
Chocolate bar
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Let's run all the steps on a chocolate bar. We have one quarter plus one half. Both bottoms fit into four, so we cut the bar into four blocks. One half is the same as two quarters. Now one block plus two blocks makes three quarters of the bar, and that can't shrink any further.
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9
Your turn
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Now it's your turn. Grab your pencil and use all four steps. Problem one is a pizza, one half plus one quarter. Problem two is a chocolate bar, one third plus one quarter. Problem three is one half plus one sixth, and don't forget to simplify at the end.
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10
Check answers
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Pencils down, let's check. One half plus one quarter is two quarters plus one quarter, so three quarters. One third plus one quarter needs twelfths, so four twelfths plus three twelfths makes seven twelfths. One half plus one sixth is four sixths, and that shrinks to two thirds. Give yourself a pizza point for every one you got.
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11
Same size
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Here is the one thing to take home. Fractions only add when the pieces are the same size. Let's do a quick recap. Check the bottoms, find a common denominator, convert, add the tops, and simplify. Do that, and you can share the pizza with anyone.
