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## Homework Statement

The initial activity of I_131 is 0.74MBq.

The half time of I_131 is 8 days.

How large is the activity after two days?

## Homework Equations

[tex] A = A_0 e^{-\lambda t} [/tex]

## The Attempt at a Solution

We know

t = 2 days

A_0 = 0.74 MBq

T_0.5 = 8 days

1. Solve the activity constant

[tex] \lambda = ln2 / T_0.5 [/tex]

2. Plug it to the equation

[tex] A = A_0 e^{(-ln2 / T_0.5) * t} [/tex]

I standardise the units to SI and then omit/cancel them

[tex] A = 0.74E6 * e^{-ln2 / 4} [/tex]

[tex] = 6.22E5 Bq [/tex]

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The right answer is 0.4 times what I get

[tex] A = 0.4 * 6.22E5 Bq [/tex]

[tex] = 250 kBq[/tex]

I am not sure where the 0.4 is got.