Where did the mental math go?

Recently I saw this kind of problem used as an example; it went something like this:

We're in the grocery store. We're going to buy 6 watermelons, and divide each of them into fourths. How many slices will we get?

I want you to think hard :) ... and find the answer to that question. Also note HOW you found it...

Got it? Now listen how it was solved in this particular mathematics tutorial:

They changed a fourth into its decimal equivalent, 0.25, and proceeded to divide 6 by 0.25 - going thru the motions of multiplying both dividend and divisor by 100 before getting to the long division of 600 ÷ 25, and then finally onto the answer!!!

And this was for 7th grade math. Sad!

What happened to mental math?
What happened to solving problems with the most efficient and quickest way?

Well here's a suggestion for an example instead of that one:
You are considering buying notebooks for your school which are $0.42 each. Your budget is $200. How many notebooks can you buy?

You can solve this by going 200 ÷ 0.42.
But even this problem you can estimate the answer this way:

Five notebooks is about $2. I have hundred times as much money so I should get a little less 500 notebooks with $200.

In fact, shopping is full of opportunities to practice estimation and mental math. Just try sometime!

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