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We know that,

Where,

A= Activity of the substance

A₀ = Initial activity

t = time

λ = decay constant

Given,

Half-life = 10h, A₀=1 Ci

Activity after 9 hours

= 0.536 Ci

Number of atoms left after 9 hours

Activity after 10 hours = 0.5 Ci [As 10h is half-life]

Number of atoms left after 10 hours

Hence, net disintegrations at the 10^th hour = (1.03×10¹⁵-9.60×10¹⁴)

= 6.92×10¹³ atoms