Removing material and waiting

A uniformly mixed radioactive sample has parent activity 1200 Bq. Half the material is removed immediately. The retained portion is then left for one parent half-life.

Find the parent activity of the retained portion at the end.

Organize the givens & assumptions

Initial parent activity: 1200 Bq
Retained material fraction: 1/2
Waiting time after removal: one half-life

Uniform concentration, no change to the decay constant, and no daughter activity counted. Removing material takes negligible time.

A $300\ \mathrm{Bq}$
B $600\ \mathrm{Bq}$
C $1200\ \mathrm{Bq}$
D $150\ \mathrm{Bq}$

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