The radioactive substance strontium-90 has a half-life of 28 years. In other words, it takes 28 years for half of a given quantity of strontium-90 to decay to a non-radioactive substance. The amount of radioactive strontium-90 still present after t years is modeled by the expression 80*2^-t/28 grams.
Evaluate the expression for t = 0, t = 28 and t = 56. What does each value of the expression represent? How would i do this equation?

Question

The radioactive substance strontium-90 has a half-life of 28 years. In other words, it takes 28 years for half of a given quantity of strontium-90 to decay to a non-radioactive substance. The amount of radioactive strontium-90 still present after t years is modeled by the expression 80*2^-t/28 grams. 
Evaluate the expression for t = 0, t = 28 and t = 56. What does each value of the expression represent? How would i do this equation?

lgbasallote · Answer

you substitute for t..

anonymous · Answer

for all of them? 0 , 28, n 56?

lgbasallote · Answer

let x be the final population...it says the equation is
$$x = 80 \times 2^{-t/28}$$
so yeah substitute t = 0, 28 and 56

i.e. 

when t = 0
$$x = 80 \times 2^0 \implies 80 \times 1 \implies 80$$
like that

anonymous · Answer

then i would do the same for 28 and 56?

lgbasallote · Answer

yup

lgbasallote · Answer

good news for you... you dont need a calculator for that =))

anonymous · Answer

okay thanks.. can i post my answer and you'll check for me to see if their right? lols.

lgbasallote · Answer

sure

anonymous · Answer

is it like this x=80*2^-0/28? or without the /28?

lgbasallote · Answer

well...0/28 is just same with 0 without /28

lgbasallote · Answer

it still becomes 0

anonymous · Answer

ok

anonymous · Answer

okay so for 80*2^-28/28 , x=80
80*2^-56/28 = 80 
Is  this right?

lgbasallote · Answer

yup..

btw just so you know...that was the example i did above hehe lol

lgbasallote · Answer

^i meant the 0

lgbasallote · Answer

wait...why is everything equal to 80?

anonymous · Answer

oh wait the 80*2^-28/28 i got x=1/16777216

anonymous · Answer

and uh i dont know why everything equals to 80..

lgbasallote · Answer

you do 2^-28/28 before multiplying by 80

anonymous · Answer

how would i put 2^28/28 into a calculator?

lgbasallote · Answer

$$\Large 80 \times 2^{-28/28} \implies 80\times 2^{-1} \implies 80 \times \frac 12$$

lgbasallote · Answer

that was why i was saying you wont need a calculator

anonymous · Answer

okay let me see if this is right.
80*2^-56/28 = 80*2^-2 ?

lgbasallote · Answer

right. simplify that further

anonymous · Answer

=20

lgbasallote · Answer

right

anonymous · Answer

so back to one of the question. What does each value of the expression represent? 
would this mean that the equations i did equals to ? represent grams?

lgbasallote · Answer

i believe it means the answer to each one

anonymous · Answer

oh, okay.. lol thank you.

lgbasallote · Answer

welcome