"348 yen to go for free shipping" - Thinking about the power of arithmetic in daily life -
When shopping online, you see a message like this.
"Spend 348 yen more to get free shipping"
Now, what should we add to the cart?
When this happens, I don't pull out a calculator.
I think about it roughly.
If I add this 216 yen item and this 152 yen item...
It's over 210 and over 150, so that's over 360. Okay, that's enough.
This split-second judgment. Actually, this is4th-grade arithmetic.
When we think "roughly," we are choosing unconsciously
Let's consider another situation.
For example, snacks for a field trip.
Suppose there is a rule of "up to 300 yen."
While choosing snacks, you calculate in your head.
124 yen and 179 yen... so 120 and 180 make 300 yen. Safe!
...Is it really safe?
124 + 179 = 303 yen.
That's 3 yen over.
If you went shopping with exactly 300 yen,
that awkward moment at the register, haha.
Some of you may have experienced this.
Why does this happen?
You might have already noticed.
At this time, you wereroundingto estimate.
You saw 124 yen as 120 yen, and 179 yen as 180 yen.
Rounding is a very excellent method for finding the number
closest to the actual value.
But,it can shift higher or lower.
So, in situations where you absolutely want to stay within 300 yen,
it is dangerous.
So, what should you do?
You should just round up.
Estimate 124 yen as "130 yen" instead of 120 yen,
179 yen as "180 yen",
and round up slightly.
If you think there is a high possibility that the total will exceed 300 yen,
you can decide not to buy it.
Conversely, if it is within 300 yen even when estimated higher, it is definitely within 300 yen in reality.
When you absolutely do not want to exceed your budget, you estimate higher to ensure safety.
This is a situation where rounding up is useful.
And now, the "free shipping" situation mentioned at the beginning.
Now, let's get back to the topic.
In the situation of "348 yen to go for free shipping,"
how should we think about it?
Should we estimate higher like before?
——No, it is the opposite.
What we are looking for in this situation is to
definitely exceed 348 yen.
If you estimate higher and think "it probably exceeds it,"
you might fall short of the amount and end up paying for shipping.
Therefore, estimate lower.
Estimate 216 yen as "210 yen" and 152 yen as "150 yen,"
and the total is still 360 yen.
By estimating lower like this, if it still exceeds the condition, then it definitely exceeds it in reality
.
This is a situation where rounding down is useful.
Even though it is the same "roughly," the approach is reversed.
This is the interesting part.
Want to keep it within 300 yen → Estimate higher (round up)
Want to definitely exceed 348 yen → Estimate lower (round down)
Even though it is the same "calculating roughly,"
you shift in the completely opposite direction depending on your goal.
As adults, we do this unconsciously.
Textbooks also cover situations where we use approximate numbers in daily life.
However, when teaching, I feel there is
another step between "being able to round" or "being able to round up/down" and
being able to choose the right method yourself according to the purpose.
I feel there is another step.
Even if children can find approximate numbers on a test, when it comes to real-life situations,
some children get confused, wondering, "Which estimation method should I use?"
"Rounding is not always the answer"
In the 4th-grade unit on "Approximate Numbers," what I want to value most is
this ability to make judgments.
Rounding, rounding up, and rounding down.
The goal is not just to memorize these three methods.
The goal is to be able to look at the purpose and decide for yourself which one to use.
348 yen until free shipping.
Snacks for the field trip must be 300 yen or less.
What we use casually as adults is exactly this power.
Second-semester lesson design, to be released this Saturday
How can we foster this "ability to choose the right method" in children?
What I will release this Saturday (8/15) is
a lesson where children discover rounding up and rounding down for themselves
through shopping for snacks..
"Let's choose snacks for the field trip within 300 yen."
"Let's get a gift by spending over 700 yen."
In one case, you "definitely don't want to go over."
In the other, you "definitely want to go over."
By placing two shopping scenarios with opposite conditions side-by-side,
it is a lesson where children realize for themselves that "the estimation method must also change."
Instead of just memorizing how to find approximate numbers,
we think about choosing the right method according to the purpose.
Let's work together to create lessons where children will think
that math is fun in the second semester!
