Let's plug in (18,10) into each answer choice until we find the one that has it as a solution.
A) 2x - y = 26
2(18) - 10 = 26
36 - 10 = 26
26 = 26
Your answer is A.
I'll just do the rest to show that you the they are incorrect.
B) x + y = 18
18 + 10 = 18
28 ≠ 18
C) 2x - 2y = 36
2(18) - 2(10) = 36
36 - 20 = 36
16 ≠ 36
D) x - y = -28
18 - 10 = -28
8 ≠ -28
Your answer is A) 2x - y = 26.
B-y is the another factor
Answer:
8978 grams
Step-by-step explanation:
The equation to find the half-life is:

N(t) = amount after the time <em>t</em>
= initial amount of substance
t = time
It is known that after a half-life there will be twice less of a substance than what it intially was. So, we can get a simplified equation that looks like this, in terms of half-lives.
or more simply 
= time of the half-life
We know that
= 35,912, t = 14,680, and
=7,340
Plug these into the equation:

Using a calculator we get:
N(t) = 8978
Therefore, after 14,680 years 8,978 grams of thorium will be left.
Hope this helps!! Ask questions if you need!!
The answer is 1.67
x 10<span>^25. Your welcome.
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