BASE MULTIPLICATION ABOVE BASE
NUMBERS JUST OVER TEN
The method used in the last section can also be used for numbers just over 10 rather than
numbers just under 10.
Suppose you want to multiply 12 and 13, which are both close to 10.
For 12 × 13 you notice the numbers are close to 10 and that 12 is 2 over ten, and 13
is 3 over ten.
So set the sum out as before except that because the numbers are over ten you put a
plus instead of a minus:
12 + 2
× 13 + 3
-----------------
15 / 6
Then you cross-add
12 + 3 OR 13+2 =15 to get the first part of the answer:
figure: 2 × 3 = 6
So 12 × 13 = 156.
NUMBERS JUST OVER HUNDRED
Multiplying numbers that are over 100 is even easier than multiplying numbers just under
100.
Suppose we want 103 × 104.
103 × 104 = 10712.
103 + 03
104 + 04
-----------------
107 / 12
The method is similar to the previous one.
103 is 3 over 100, so put +3 next to it.
And 104 is 4 over 100 so put +4 next to it.
Then 103 + 4 = 107 or 104 + 3 = 107,
and 4 × 3 = 12.
So now we cross-add, and multiply vertically.
PRACTICE
a 107 × 104 b 107 × 108 c 133 × 103 d 102 × 104 e 123 × 102 f 171 × 101 g 103 × 111 h 125 × 105 i 103 × 103 j 111 × 111 k 162 × 102 l 113 × 105
m 1 0 3
? ? ?
----------------
1 0 8 1 5 find the missing numbers
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