Draw a plain scale for a map whose scale is 1 cm : 10 km.

 1 cm on the map represents 10 km on the ground.

15 cm on the map represents on the ground

= 10 × 15 = 150 km.

(The normal length of scale should be 15 cm) 150 km is a round number. Therefore we not select any other number.

Scale 1 cm : 10 km.

Construction. Draw a line 15 cm long. Divide it into 15 primary divisions. Each division represents 10 km. Sub-divide the left-hand divisions into five secondary divisions, each showing a distance of 2 kms.