Topographical sheet numbers, such as 71E/NW, follow a standardized system that indicates both location and scale. The standard scale for topographical sheets designated with this type of numbering convention is typically 1:125,000.
The scale 1:125,000 means that one unit of measurement on the map represents 125,000 of the same units on the ground.
$ \frac{125,000 \text{ inches}}{63,360 \text{ inches/mile}} \approx 1.97 \text{ miles} $
Therefore, the scale of topographical sheet No. 71E/NW is 1 inch to 2 miles.