Missing numbers in addition and subtraction | 2nd grade | Khan Academy

TL;DR

Learn quick mental math tricks to find the difference between two numbers by adding or subtracting numbers that make the calculations easier.

Transcript

• Let's say someone walks up to you on the street and says, "Quick! "73 plus blank is equal to 57." What would blank be? Well, there's a couple of ways to think about it. Blank is essentially what you have to add to 57 to get to 73. It's the difference between 57 and 73. And so you could think about it as, "Well, let me start with 57..." And this i... Read More

Key Insights

• #️⃣ One approach to finding the missing number in addition or subtraction problems is to start with the known number and add or subtract numbers that make the calculations easier.
• #️⃣ Another approach is to think about the missing number as the difference between two numbers or as the sum of the known number and the numbers added.
• 🤗 These mental math tricks can be useful in situations where calculating the missing number mentally is more convenient than using pen and paper.
• 🅰️ Understanding the relationship between addition and subtraction helps in solving these types of problems.
• #️⃣ Number lines can be helpful visual representations to understand the relationship between numbers and the operations needed to find the missing number.
• 👻 Multiple approaches can be used to solve these problems, allowing for flexibility and individual preferences.
• 🐎 Practice and familiarity with these mental math strategies can improve speed and accuracy in solving these types of problems.

Questions & Answers

Q: How can I find the missing number in an addition problem?

To find the missing number in an addition problem, start with the known number and add numbers that get you to nice round numbers until you reach the desired sum. Keep track of the numbers you added to find the missing number.

Q: Is there another way to find the missing number in an addition problem?

Yes, you can also think about the missing number as the difference between the sum and the known number. Write the equation as the known number plus the missing number equals the sum. Then solve for the missing number using basic arithmetic.

Q: How can I find the missing number in a subtraction problem?

To find the missing number in a subtraction problem, start with the desired difference and subtract numbers that make the math easier until you reach the known number. Keep track of the numbers you subtracted to find the missing number.

Q: Can I solve a subtraction problem by adding instead of subtracting?

Yes, you can also approach a subtraction problem by adding numbers to the known number. Start with the known number and add until you reach the desired difference. The sum of the known number and the numbers you added will be the missing number.

Summary & Key Takeaways

• The speaker explains how to find the missing number in an addition problem by starting with the known number and adding until reaching the desired sum.

• Similarly, the speaker demonstrates how to find the missing number in a subtraction problem by starting with the desired difference and subtracting until reaching the known number.

• The speaker also explains how to approach these calculations by thinking about the difference between two numbers or by adding/subtracting to find the unknown number.