Answer:
Rule of 9 : an integer is divisible by 9 if and only if the sum of its integers is divisible by 9.
Proof :
Let us consider a number x such that,
Where, are the the digits of the number x,
So, we can write,
Let,
Since, a number is in the form of , where k is an positive integer, is always divisible by 9,
⇒ are divisible by 9.
⇒ x-S is divisible by 9,
⇒ If S is divisible by 9 ⇒ x must divisible by 9,
Or if x is divisible by 9 ⇒ S must divisible by 9.
Hence, proved...
We have been given that last month Maria purchased a new cell phone for $500. The store manager told her that her cell phone would depreciate by 70% every 6 months.
We know that an exponential function is in form , where,
y = Final value,
a = Initial value,
r = Decay rate in decimal form,
x = Time in years.
Let us convert into decimal form.
Initial value of car is 500, so .
Since value of phone depreciates every months, so value of phone will depreciate twice in a year.
Upon substituting our given values in exponential decay function, we will get:
To find the value of phone after 2 years, we will substitute in our equation.
Therefore, option D is the correct choice.
Let us simplify our equation.
Therefore, option B is correct as well.