Answer:

Step-by-step explanation:
The exponential decrease formula that we must use is:

Where A is the amount of thorium as a function of time
is the initial amount and k is the decrease rate
We know that initially there are 50 gr and that you decay 50% every 1.9 years.
So


We solve the equation for k





After 22.8 years the amount of thorium that exists is:


B. Bc rows are horizontal while columns are vertical.
Let $40 = V1
Let $50 = V2
(V2 -V1)/ |V1| x 100 = ?
(50 - 40)/ |40| x 100 = 25%
The percent of change is 25%