How would you answer this 1st grade test question. Explain your reasoning.

[Threedifferentones]

180. This IS simple math. 14-6! Except they are teaching a specific way to calculate inside one's head.

This is basically just a formalization of what myself and most of the more clever students realized on our own as kids: when you do arithmetic in your head, it is best to reduce the problem to a serious of more simple calculations. Of course for an adult 14-6 is obviously 8. But with just slightly bigger numbers, I do the same thing this system would do, albeit without any formal concept of a "ten box" or whatever.

If someone says "what's 124 minus 47," I first subtract 4 in my head to get the number to an even 120, then I subtract the 40 to get to 80, and then I subtract the remaining 3 to get to 77.

Similarly, if someone asked me to add 47 and 77, I would first calculate that 40 and 70 make 110, and then that 7 and 7 make 14, and then that 110 and 14 make 124.

In my life I have met many educated people who cannot do that sort of arithmetic in their head, even though they easily could with pen and paper. I have realized that's because they never figured out these little mental shortcuts, which I assume is why schools are now formalizing them and teaching them to kids early.