Undo the wrong operation

In the last lesson, we crossed out the items that two purchases both had. Today, we will fix a wrong calculation by undoing it first. Mei was asked to multiply a number by 4. Instead, Mei multiplied it by 3 and got 36. What should the correct result be? We do not know Mei's number, so we draw it as 1 unit. Mei multiplied it by 3, so she got 3 units of the number, which is 36. She should have multiplied the same number by 4, which is 4 units of the number. To find the number, we undo what Mei did. She multiplied by 3, so we divide 36 by 3. 36 divided by 3 is 12. So the number is 12. To check, 12 times 3 is 36, the same as Mei got. Now we do the right calculation with 12. 12 times 4 is 48. So the correct result is 48.

Kavin was asked to multiply a number by 6. Instead, Kavin multiplied it by 4 and got 28. What should the correct result be? Try it first. Pause if you like. We do not know Kavin's number, so we draw it as 1 unit. Kavin multiplied it by 4, so he got 4 units of the number, which is 28. He should have multiplied the same number by 6, which is 6 units of the number. To find the number, we undo what Kavin did. He multiplied by 4, so we divide 28 by 4. 28 divided by 4 is 7. So the number is 7. To check, 7 times 4 is 28, the same as Kavin got. Now we do the right calculation with 7. 7 times 6 is 42. So the correct result is 42.

Siti was asked to multiply a number by 4. Instead, Siti divided it by 2 and got 9. What should the correct result be? Try it first. Pause if you like. We do not know Siti's number, so we draw it as a bar. Siti divided it by 2, so the number is cut into 2 equal parts, and 1 part is 9. She should have multiplied the same number by 4, which is 4 units of the number. To find the number, we undo what Siti did. She divided by 2, so we multiply 9 by 2. 9 times 2 is 18. So the number is 18. To check, 18 divided by 2 is 9, the same as Siti got. Now we do the right calculation with 18. 18 times 4 is 72. So the correct result is 72. First, undo the wrong calculation to find the number. Then do the right calculation.