A scientist measures the initial amount of Carbon-14 in a substance to be 25 grams.
The relationship between A, the amount of Carbon-14 remaining in that substance, in grams, and t, the elapsed
time, in years, since the initial measurement is modeled by the following equation.
A= 25e -0.000124t
In how many years will the substance contain exactly 20 grams(g) of Carbon-14?
Give an exact answer expressed as a natural logarithm.
years
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In 386750.36 years the Carbon - 14 will be 20 g in the substance

Step-by-step explanation:

Here A = 20 g

The given equation is,

A= 25e - 0.000124t

We need to find the value of t

20 = 25e - 0.000124t

20 - 25e = - 0.000124t

25e - 20 = 0.000124t

(25e - 20 ) / 0.000124 = t

t = (25e - 20) / 0.000124

t = (65.957 - 20) / 0.000124

t = 45.957 / 0.000124

t = 386750.36 years

256π units²   (answer b)

step-by-step explanation:

the formula for the surface area of a sphere of radius r

is             a = 4πr²

and so, if r = 8 units, then the surface area is:

a = 4π(8 units)² = 256π units²   (answer b)