# Mental math tricks and tips | Kathryn Re

Mental math and estimation are two topics that have been ignored or minimized in schools. If you graduated from high school before 1990, your math education was mostly computation and other processes (mathematicians call these algorithms). That left little time for other math topics. If you are still in school or graduated after 1990, your math education was expanded to include probability, statistics, geometry and problem solving. Even with the general use of calculators to speed up our learning of all the new material, mental math and estimation were again put on the back burner and left there.

Mental math and estimation are two topics that have been ignored or minimized in schools. If you graduated from high school before 1990, your math education was mostly computation and other processes (mathematicians call these algorithms). That left little time for other math topics. If you are still in school or graduated after 1990, your math education was expanded to include probability, statistics, geometry and problem solving.  Even with the general use of calculators to speed up our learning of all the new material, mental math and estimation were again put on the back burner and left there.

Let’s try another one. Let’s use our new strategy to add 17+7.  We think of 17 as 10+7, so we have 10+7+7. The last 7 is really 3+4 (because we had to add something to 7 to get 10). We now have 17+7 is really 10+7+3+4. 7+3 is 10, and 10 added to 10 is 20. Now add the 4 to get 24. Notice that we never really had to “carry” anything.

If you can add 10s to any number, then you can add 9s, too. Since 9 is really one less than 10, we can add a 9 by adding 10, then backing up one.  For example, 23+9 can be thought of as 23+10-1, or 33–1, which is 32.

One last example: At the dinner table, your middle school student asks you what the result of 27+48 is. With your hands occupied with the knife and fork, you can’t do the problem “in the air” and, with the calculator in the next room, you may have to admit to your student that you don’t know the answer. But wait! You now have the strategy of substitution!  Think of 27 as 20+7 and 48 as 40+8. Add the tens first: 20+40 is 60. So now you only need to deal with 7+8.  You know that 7+3 is 10, so you think of 7+8 as 7+3+5 which is 15. So 60+15 is 60+10+5=75.  Again you never had to “carry” anything!

See if you can do these “in your head”.  Then check the answers at the bottom of the column.

1)  37 + 17        2)  19 + 57        3)  286 + 428

————————————–

(1)  37 + 17 = 30 + 10 + 7 + 7 = 40 + 7 + 3 + 4 = 50 + 4 = 54

(2)  19 + 57 = 57 + 19 = 57 + 10 + 9 = 67 + 9 = 67 + 10 – 1 = 77 – 1 = 76

(3)  286 + 428 = 200 + 400 + 80 + 20 + 6 + 8 = 600 + 100 + 6 + 8 = 700 + 6 + 4 + 4 = 714

### More in Life

##### East Pierce hosting annual open house Sept. 29

Don’t miss out on the free, family fun activities and demonstrations, like tearing a car apart with the jaws of life.

##### Fun run will benefit animal rescue group

Grab your furry friend’s leash and head out to Lake Wilderness Park on Sept. 29.

##### Some days, it’s good to feel needed

Some days, you just need a hug. Other days, you only want… Continue reading

##### Local Scout earns his wings

Kyle Ross Dunning is a member of Boy Scout Troop 422.

##### Beautify Enumclaw, Buckley | Slideshow

Many public spaces, like Enumclaw’s community garden by the library and Veterans Memorial Park, or Buckley’s Youth Center, were given makeovers by scored of energetic volunteers.

##### Enumclaw lessons helped lead to military success | Navy Outreach

“Growing up in a small town, it kind of helped me gauge different people and network in a way that would benefit me,” Nelsen said. “It is humbling growing up in a small town and I did not lose sight of where I came from.”

##### Flu season is coming; get your shots soon | Public Health Center

To help you decide when, where, and how to get vaccinated, Public Health Insider compiled answers to some of the most common questions they see regarding to flu vaccinations.

##### Answers may be hard to come

You’re on the edge of your chair. Curiosity is almost killing you;… Continue reading

##### Falls prevention event highlights awareness, action | Pierce County

This month is National Fall Prevention Month.

##### Enumclaw welcomes new faces in schools, district office

Check out the new educators and other school staff.

##### Levy money to aid senior programs in Enumclaw, Black Diamond | King County

By 2040, more than a quarter of King County’s population will be seniors. Healthy lifestyles and social engagement are keys to living long and living well.

##### Program designed for families dealing with mental illness | Rainier Foothills Wellness Foundation

The classes will focus on several different kinds of mental illnesses and the skills necessary to handle crisis situations, how to listen and communicate, and self-care for dealing with worry and stress.