What time was the math problem in the first grade of primary school an hour ago? There are three round clocks, the first is twelve o'clock sharp, the second is seven o'clock sharp, and the third is eleven o'clock sharp. The first one is exactly eleven o'clock, the second one is exactly six o'clock and the third one is exactly ten o'clock.

The first one is exactly 12. An hour ago, it was12-1=11,which is exactly eleven o'clock.

The second one is seven o'clock sharp. It was 7- 1=6 an hour ago, which is exactly six o'clock.

The third is exactly eleven o'clock. It was11-1=10 an hour ago, which is exactly ten o'clock.

Nature:

Subtraction follows several important patterns. It is anti-commutative, which means that changing the order will change the sign of the answer. It is not associative law, that is, when a subtraction exceeds two numbers, the order of subtraction is very important. Subtracting 0 does not change a number.

Subtraction also follows predictable rules related to addition and multiplication. These laws can be proved, starting with the subtraction of integers and summarizing them with real numbers and other things.