Pls tell me which value has been substituted in underlined step...0.18*10^-4?? Where does it come from

Dear student

As given in question, we have to calculate average force exerted on the surface in 30 minutes time span.
So, in underline place time value is being put
As 30 minutes =30x60=1800seconds
$$\begin{gathered} It implies 30 minutes =1800seconds \\ \Rightarrow 30 minutes =0.18\times {10}^4 seconds \\ So in underlined place above time value is being put in orde to calculate average \\ force \end{gathered}$$

​​​​​​
Regards