Answer:
The original mass of the substance was 10g.
Step-by-step explanation:
The half-life of a radioactive substance is the amount of time it takes for half of the substance to decay. In this case, the half-life is 5 hours.
We can use the half-life formula to find the original mass of the substance:
N = N0 * (1/2)^(t/T)
where:
---N0 is the initial mass of the substance
---N is the remaining mass of the substance after time t
---T is the half-life of the substance
We know that after 20 hours, only half of the substance remains:
N = N0 * (1/2)^(20/5) = 0.5 * N0
If we solve for N0, we get:
N0 = N / 0.5 = 5g / 0.5 = 10g
Therefore, the original mass of the substance was 10g.