Out of $300 \mathrm{g}$ substance [decomposes as per $1^{\text{st }}$ order], how much will remain after $18\mathrm{hr}$ ?
$\left({\mathrm{t}}_{0.5}=3\mathrm{hr}\right)$

No. of Half life's gone = 18/3 = 6

N₀ = 300 gms

N_t = N₀ $\times$(1/2)ⁿ 

N_t = 300 $\times$($\frac{1}{2}$)⁶

N_t = 300 $\times$ $\frac{1}{64}$= 4.68 g