The data of the problem can be summarized as follows:

NA = 2ND     ---1

lA = 1.5lD    ---2

Wanted is the time when NA = ND

N0Ae-(lA)t = N0De-(lD)t

Substitute 2 and 1 to get

ln2 -1.5lDt = -lDt

t = 19 day

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