Natural numbers are closed under division: false.
A set is closed under a certain operation if the results of that operation are always inside that set.
So, if natural numbers were closed under division, the division of two natural numbers would always be a natural number.
You have plenty of counterexamples, to pick one you may divide any odd number by 2: 5/2 is not a natural number.
Negative numbers are closed under addition: true.
Let 
be two positive numbers. So, 
are two negative numbers. Their sum is
And since 
is positive, we deduce that 
is negative, so the sum of two negative numbers is still negative.
Prime numbers are closed under subtraction: false.
This would mean that the subtraction of two primes is also a prime. Again, there are many counterexamples: 7 is prime and so is 3, but their difference 7-3 is 4, which is not prime.
He rents for d days, but two days are free, so he pays for only d - 2 days.
The rental fee is $45 per day, so for d - 2 days, he pays 45(d - 2). The total he pays is $315, so the equation is
<u>45</u>(<u>d</u> - <u>2</u>) = <u>315</u>
Now we solve the equation to find d, the number of days.
45(d - 2) = 315
45d - 90 = 315
45d = 405
d = 9
The number of days is 9.
Answer:
6/18
Step-by-step explanation:
Total amount of money that they have:
5 + 7 + 6 = 18
Since Dina has 6, the answer should be 6/18.
Answer:
70. 8x⁴
71. 3n - 10
72. a/5 + 12
Step-by-step explanation:
70. a number "x" raised to the fourth = x⁴
Product of 8 and x⁴ = 8 × x⁴
= 8x⁴
71. 3 times a number "n" = 3 × n = 3n
3n decreased by 10 = 3n - 10
72. Quotient of a number "a" and 5 = a/5
12 more than a/5 = a/5 + 12
