About this video
Alan has 5 marbles, finds 4 more, then triples his marbles. The video writes this as the numerical expression 3 x (4 + 5) without working it out, using parentheses so the adding happens before the tripling. Evaluating it gives 9 marbles before tripling and 27 after.
You'll be able to
- write a numerical expression that models a word problem.
- use parentheses to show which operation happens first.
- tell the difference between writing an expression and evaluating it.
Key words
- numerical expression
- A group of numbers and operation signs, like 3 x (4 + 5), with no equals sign.
- parentheses
- Brackets ( ) that show which part to work out first.
- triple
- To multiply by 3.
Transcript
Alan found 4 marbles to add to his 5 marbles currently in his pocket. He then had a competition with his friends and tripled his marbles. Write a numerical expression to model the situation without performing any operations. So let's think about what's going on.
So he already had 5 marbles in his pocket. And then he found 4 more marbles to add to that. So we can add the 4 marbles to the 5 marbles. So 4 marbles plus the 5 marbles. So that's what happens after the first sentence. He found 4 marbles to add to his 5 marbles currently in his pocket.
He then had a competition with his friends and tripled his marbles. So this is how many marbles he had before tripling. And now he's tripling his marbles. So we want to multiply 3 times the total number of marbles he has now-- times 4 plus 5. So this right over here is the numerical expression that models the situation without performing any operations.
We, of course, could then actually calculate this. He has 9 marbles before tripling. And then you multiply it by 3 and he has 27. But this is what they're asking for. They want us to write this expression.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.