it is given that hermie used 1 inch of clay to represent the 5 miles of the rivers actual length.
As the total clay used is 50 inches
so the actual length of the river will be
[tex]\text{total clay used }\times\text{river's actual length in 1 inch}[/tex][tex]50\times5=250\text{ miles}[/tex]so the river is 250 miles long.