Multiplication by 4

Doubling a two-digit number is the only skill needed to answer this question mentally in 3 seconds. Since 4 = 2 x 2. Young children who still do not know their times 4 table can use doubling twice to get the result.

This is discussed in Chapter 5 of my book “25 Math Short Cuts (available at https://www.facebook.com/MATHInicPhils for only P300). As what we have written before, doubling is also useful in multiplying by 4 and by 8 and in dividing by 5, 25 and 125.

Mastering doubling is also very helpful in understanding the binary system which is the basis of the computers and calculators that we now use.

Applications of doubling and its opposite, halving, are found in many questions in Vedic Math Olympiads. (For more tips on VMO questions, follow us at  https://www.facebook.com/MATHInicPhils or join our group Philippine National Vedic Mathematics Olympiad at https://www.facebook.com/groups/419302608636234.

To solve this problem in 3 seconds, we can

1. Mentally partition the number into 35|14| 23
2. Double each part to get 70|28|46
3. Double again to get 140 | 56 |92

Practice exercises

1. 12 x 4 =
2. 21 x 4 =
3. 142 x 4 =
4. 317 x 4 =
5. 1123 x 4 =
6. 3245 x 4 =
7. 21314 x 4 =
8. 23145 x 4 =
9. 342356 x 4 =
10. 476324 x 4 =

Answers to last week’s exercises.

1. 23 x 11 = 253
2. 63 x 11 = 693
3. 234 x 11 =2574
4. 725 x 11 =7975
5. 5261 x 11= 57871
6. 7163 x 11 = 78793
7. 52361 x 11 = 575971
8. 71635 x 11= 787985
9. 8080808 x 11 =88888888
10. 12121212121 x 11 = 133333333331