Kids should learn both: precision and accuracy, and ballpark; there're places for each. When you go food shopping with exactly $80, it's very helpful to be able to approximate the price of what's in your cart. When travelling cross country, it's helpful to have an idea how far you can go before you need to refuel which will also tell you about where you'll be when you need fuel. When computing a tip, take the total, shift the decimal place to the left one place, add about half that again, then round up: close enough to 15% plus a little. For most everyday living, ballpark math is good enough.
But for many other things, accuracy and precision are paramount. Construction engineering relies on exacting math (well, outside of graft and corruption). Retail sales/change must be accurate. Band counts and bacteria counts per unit area or per unit volume must be accurate for doctors to make accurate diagnoses.
I taught myself ballpark math back in grade school and use it most of the time, and I use accurate and precise math when and where it's needed. It's useless to me to know that my '98 gets 19.6349586736294 miles per gallon. But it's good to know that I can travel about 600 miles before I'll need to refuel; from here, that would put be in Ann Arbor or north of Indy.
I guess it really comes down to how accurate computations need to be and how precise (number of decimal places) they need to be. And kids should learn both ways.
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