# There are as many notes as number of oxygen atoms in 24.8g $Na_2S_2O_3.5H_2O$ (molecular weight =$248gmol^{-1})$.A note-counting machine counts 48 million notes per day.Days taken to count these notes are

$\begin{array}{1 1}(a)\;10^{17}& (b)\;10^{16} \\(c)\;10^{10} &(d)\;10^{12}\end{array}$

No of moles =$\large\frac{24.8}{248}$=0.1mol
O-atoms=$8\times 6.023\times 10^{23}=48\times 10^{23}$
Days=$\large\frac{48\times 10^{23}}{48\times 10^{6}}$$=10^{17}$
Hence (a) $10^{17}$ is the correct answer.