Question: Division is scarcely taught in my child's school. I'd like to teach her how to solve division using mental math. How should I do this? -- For Division
Answer: Being able to handle division in your head is a worthwhile skill for both children and adults. Just think of being able to divide a restaurant bill between five people as an example. Or having your child be able to easily divide 100 marbles between nine children.
Start your child out with one-digit division using this technique advocated by Arthur Benjamin and Michael Shermer. The first step for your child is to figure out how many digits will be in the answer. For example, to solve 140 divided by 6, you would first determine that 140 lies between 6 x 10 = 60 and 6 x 100 = 600. So the answer will lie between 10 and 100 and will roughly be a two-digit number. The next step is to figure out the largest multiple of 10 that can be multiplied by 6 with an answer below 140. 6 x 20 = 120 and 6 x 100 = 600, so the answer must be in the 20s. Then the number 120 is subtracted from 140 to get 20. Now the actual division problem is 20 divided by 6. Since 6 x 3 = 18, which is 2 away from 20, you have the rest of the answer, which is 23 with a remainder of 2.
Becoming quick at mental division takes time. Your child will need to practice doing this with similar problems. Ultimately what is happening is that you are using the process of simplification to make the problem simpler. This method of mental division and easy tricks for adding, subtracting and multiplying are explained in the book "Secrets of Mental Math," by Arthur Benjamin and Michael Shermer. Using this book will turn your daughter into a math whiz in no time. Soon she'll be doing all kinds of calculations amazingly fast.