Why '5+5+5=15' is wrong under the Common Core

[rangerrebew]

Why '5+5+5=15' is wrong under the Common Core

 

Here's a "repeated addition" Common Core problem that's taught in third grade in US schools:

Use the repeated-addition strategy to solve: 5x3

If you answer the question with "5+5+5=15,” you would be wrong.

The correct answer is "3+3+3+3+3.”

Mathematically, both are correct. But under Common Core, you're supposed to read "5x3" as "five groups of three." So "three groups of five" is wrong.

According to Common Core defenders, this method will be useful when students do more advanced math. This way of reading things, for instance, can be used when students learn matrices in multivariable calculus in high school.

But parents aren't happy about it.

http://www.businessinsider.com/why-55515-is-wrong-under-the-common-core-2015-10

[jmyrlefuller]

Multiplication is commutative. It doesn't matter which direction they're grouped; the end result is the same.

[Oceander]

Never saw that, ever.  Either the fuller context is missing, or this is just one more example of an incompetent teacher.  In fact Occam's Razor suggests that the simpler, and therefore more likely, explanation is an incompetent teacher given the fact that teachers' unions have made it almost impossible to fire incompetent teachers and tend to force competent teachers out because they must sit around and wait while the incompetents who have seniority get all the pay and benefits.