To find the time at which only 1 mg remains, we need to solve the equation
, where
represents time.
Let's solve for
:
.
Dividing both sides of the equation by 40:
.
Subtracting 2 from both sides:
.
Multiplying both sides by 30:
.
Simplifying:
.
Therefore, the time at which only 1 mg remains is
.

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