Multiplying 3-digit by 2-digit numbers: Error analysis | Grade 5 (TX TEKS) | Khan Academy

So we have a situation here where someone is attempting to multiply 586 * 43, and what we want to do together is figure out if they did this correctly or whether they made a mistake. And if they made a mistake, what step did they make a mistake on? Actually, why don't you pause this video and have a go at this on your own before we work through this together?

All right, now let's work through this together. So let's start at the beginning here. We have 3 * 6 is 18, so you put the eight here, regroup the one. That all looks good right now. Then you have 3 * 8 is 24 plus that one that we just regrouped is 25. Put the five here, regroup the two. All looking good so far.

3 * 5 is 15 + 2 is 17. Wrote the 17 here, so step one all looks good to me. Now let's look at; we're now going to, now let's move on to the four, which is really a 40. So we're going to put a zero right over here, and then we're going to say 4 * 6 is 24. Put the four here, regroup the two.

4 * 8 is 32 + 2 is 34. Put the four, regroup the three. 4 * 5 is 20 plus the 3 is 23. So it looks like a mistake was made in step two, and of course step three is going to break down because they forgot to put the three right over there. So definitely a mistake, and the mistake is in step two.