Number Play

2, 1, 5

1. In the divisibility-by-11 method, the excess comes from place values that are 1 more than a multiple of 11. 2. These are the units, hundreds, and ten-thousands places. 3. For 320185, those digits are 5, 1, and 2. 4. So the correct digits are 2, 1, and 5.

3, 0, 8

1. In the divisibility-by-11 method, the short comes from place values that are 1 less than a multiple of 11. 2. These are the tens, thousands, and hundred-thousands places. 3. For 320185, those digits are 3, 0, and 8. 4. So the correct digits are 3, 0, and 8.

Change +b to -b

1. Compare a + b - c - d with a - b - c - d. 2. The change is (a + b - c - d) - (a - b - c - d). 3. This simplifies to 2b. 4. Since 2b is even, changing +b to -b changes the value by an even number.

Even

1. 43 and 37 are both odd, so 43 + 37 is even. 2. 672 and 348 are both even, so 672 + 348 is even. 3. Subtracting an even number from an even number gives an even number. 4. So 43 + 37 - 672 - 348 is even.