Find the capacity of a banquet hall if only 4/5 of it is full and there are 300 guests.

Dear student, 

Since the banquet is 4/5th full and occupies 300 guests,
Let its total capacity be X, therefore, X is 
$$\begin{gathered} \frac{4}{5}\times x=300 \\ x=\frac{5}{4}\times 300 \\ x=375 guests \end{gathered}$$
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Therefore the total capacity of a banquet will be 375 persons.

Regards