A parent nuclide X (half-life 20h) decays to a radioactive daughter Y (half-life 5h) which in turn decays to a stable product. Estimate the time, measured from the instant the sample is pure X, at which the activity of Y reaches its peak value.
a) Approximately 9.6h
b) Approximately 13h
c) Approximately 6.7h
d) Approximately 10h