in 1983 about 17,000,000 visitor hours were spent on boating. to the nearest million about how mant hours were spent on fishing?

Associative property and commutative property true for addition, not true for subtraction.

Step-by-step explanation:

Associative property and commutative property true for addition.

i) a + (b + c) = (a + b) + c  ---> Associative property of addition

ii) a + b = b + a --> commutative property of addition.

These properties not true for subtraction.

That's the reason Juan used additive inverse before applying these property.

Hope this will helpful.

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