Track quiz
Show on a big screenTen ideas, ten checks. The table comes first.
| Idea | How it goes |
|---|---|
| General form | For two digits, number plus reverse is 11 times digit sum. A number is 10a + b. |
| Three digits | Bigger minus smaller: the middle digit drops out. The difference is 99 times the outer gap. |
| Eleven test | Alternate plus and minus from the ones digit. Zero or a multiple of 11 passes. |
| Nine-check | If the marks disagree, the answer is wrong. If they agree, it might be right. |
| Letters for digits | A letter is one digit. A B means 10A + B, never A × B. |
| Hundred grid | One step right adds 1. One step down adds 10. |
| Saying why | Write both numbers, do the sum, take out the common factor. |
| End digits | For 2, 5 or 10, look at the last digit. For 4, look at the last two digits. For 8, look at the last three. |
| Digit sums | A number divides by 3 when its digit sum does. A number divides by 6 when 2 and 3 both divide it. |
| Two tests for 40 | To test 40, check 5 and 8. Checking 4 and 10 is not enough, because 20 passes both. |
A two-digit number is added to its own reverse. What is the total always a multiple of?
A plate reads 851. What is 851 minus its reverse?
Is 407 a multiple of 11?
The nine-check passes on a written answer. What does that tell you?
On a board, A B + 2 8 = 9 1. What is the two-digit number A B?
On a 1-to-100 grid, which way do multiples of 9 go?
10a + b plus its reverse collects to 11a + 11b. Which factor shows the sum is a multiple of 11?
Which of these numbers divides by 8? All three are even.
Which number divides by 6?
A number divides by 4 and by 10. What must be true?
Part I, Chapter 5, Number Play, is in your book. You have taken numbers apart and tested them. You have also argued about all numbers at once.
Next: We Distribute, Yet Things Multiply. It opens brackets with a letter inside.
Reveal every answer to finish this lesson.