Why is their a big emphasis placed on Estimation and Mental Math?

Why is their a big emphasis placed on
Estimation and Mental Math?
In a survey, it was discovered that as adults, estimation and mental
math are the forms used most often on a day-to-day basis.
Survey results of how adults do Math on a daily basis:
Mental Computation
Written Math
Use Calculators
Use physical objects
When you think of your own day, how do you use math? When you
are shopping, do you carry paper and pencil and keep a running total
of the amount you have spent? Do you carry a calculator to do this
task? Do you keep a running total in your head? Is this mental
recording an exact amount or an estimate?
Another example is when you are planning your day. Do you sit
down and write out how far you need to go to get to work? Do you
calculate how long it will take and what time you will need to leave
using paper and pencil? Do you use a calculator? Do you do it in
your head?
The truth is most of the math we do is actually done as mental math
or estimation, so we are trying to work on those skills in school
because we feel they are vital for success in life. The think aloud
work the students do in the Number Talk routines is a great way
to work on this skill and share the methods that different students
used. It is a chance to learn from each other.
A great activity for fostering flexibility with numbers and looking for
friendly ways to put numbers together is the activity called the Five
Five Tower Game
You need counters and either two dice or a deck of cards.
Roll the dice or pick a card and take that many counters (blocks or cubes work
best because students can make towers, if you don’t have blocks or cubes lay
counters out in a row). Repeat this until you have five towers or rows. Record
how many you have in each. Example 5 + 2 + 10 + 9 + 8 = __. Join rows or
columns together. Encourage the students to look for friendly numbers (ones
that join together to ten). Use the Ten Frame to collect the counters and then
record how many in all. Example using above numbers:
10 + (8+2) + 9 + 5 -> 10 + 10 + 9 + 5 -> 20 + (9 + 5) -> 20 + 14 -> 34 in
all. Note this is just one way to join them. Encourage students to experiment
with a variety of ways. Talk about which ones were easiest and why.