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Eg. :Consider 72 × 15

72 ÷ 2 = 36

72 + 36 = 108

Therefore. 72 × 15 = 1080

To multiply an odd number by 15, write that odd number, add half of its

predecessor and put 5 as unit digit to that added value.

Alternate method:

Divide the number by 2. Leave the remainder and add the quotient to the

given number. Attach 5 to the right of the sum.

Eg : Consider 987 × 15

987 ÷ 2 = 493

987 + 493 = 1480

Therefore, 987 × 15 = 14805

d. To divide a number by 5, multiply that number by 2 and divide it by 10.
To divide a number by 25, multiply that number by 4 and divide it by 100.

All the above mentioned ideas could be explained to the child orally initially. After

the child acquires the basic ideas regarding the four fundamental operations, a few

exercises involving addition, subtraction, multiplication and division could be given

to the child to perform. As the child is going to perform the basic operations with the

aid of the abacus, knowledge of abacus becomes a pre-requisite for effective learning

of the four basic operations. The above shortcut methods will also help in augmenting

the mental arithmetic abilities of the child.