Why the extra column is 14: The number of squares in a column is always the depth of the block. Here the block is 14 rows deep, so every column — yellow or green — holds 14 squares.
The rule in one line: Go to the nearest ten or hundred, then add or take away as many whole groups as you overshot or fell short.
Q3.
(c) Give 5 examples of problems where you can use the nearest multiple to find the product easily. Find the products as well.
Answer
Sample answer: choose problems where one number is just below or just above a ten, a hundred or a nice multiple.
My example
Nearest multiple
Adjust
Product
9 × 32
10 × 32 = 320
− 32
288
99 × 7
100 × 7 = 700
− 7
693
21 × 15
20 × 15 = 300
+ 15
315
49 × 6
50 × 6 = 300
− 6
294
101 × 8
100 × 8 = 800
+ 8
808
What makes a good example: One number should sit within 1 or 2 of a friendly number like 10, 20, 50, 100. Then you only have to add or take away one or two groups.
Check one: 49 × 6 = 294. Test it the long way: 40 × 6 = 240 and 9 × 6 = 54, and 240 + 54 = 294 ✓
Q4.
(d) Find the products of the following numbers by finding the nearest multiple. 1) 7 × 52 2) 12 × 28 3) 75 × 31 4) 99 × 15 5) 8 × 25 6) 22 × 42
Answer
All six, with the friendly number named each time.
Problem
Nearest multiple
Adjustment
Answer
1) 7 × 52
7 × 50 = 350
+ 7 × 2 = 14
364
2) 12 × 28
12 × 30 = 360
− 12 × 2 = 24
336
3) 75 × 31
75 × 30 = 2,250
+ 75
2,325
4) 99 × 15
100 × 15 = 1,500
− 15
1,485
5) 8 × 25
10 × 25 = 250
− 2 × 25 = 50
200
6) 22 × 42
22 × 40 = 880
+ 22 × 2 = 44
924
3) 75 × 31 Step 1. 31 is one more than 30. Step 2. 75 × 30 = 75 × 3 × 10 = 225 × 10 = 2,250 Step 3. Add one more group of 75. Step 4. 2,250 + 75 = 2,325
6) 22 × 42 Step 1. 42 is two more than 40. Step 2. 22 × 40 = 22 × 4 × 10 = 88 × 10 = 880 Step 3. Two more groups of 22 = 44 Step 4. 880 + 44 = 924