Question

the map of a rectangular field drawn to a scale of 1:20000 measures 20cm by 15cm. calculate the actual area of the field in sq km

Answer 1

well to start convert the 2.7 cm into km
1 km= 1000m
1 m = 100 cm
therefore 
1 km= 1000× 100
= 100000 cm 
therefore 
100000 cm =1 km
but 
1 cm = 1100000 km
therefore 
2.7 cm= 1100000× 2.7
= 2.7 ×10−5 km
therefore using a personal trick
map:actual
1 km : 20000 km
↓: ↓
2.7×10−5 km: 20000×2.7×10−5 km
=2.7×10−5 km: 0.54 km
therefore the actual distance between these 2 points is 0.54 km

Answer 2

1 cm on map = 20000 cm on the earths surface
Area on map =(20 X 15) cm
=300cm square
Therefore 300cm square on map represents
(20000 X 300) =6000000 cm square
1 Km square = 10000000000 cm square
Therefore 6000000 cm square =
6000000/10000000000
=3/5000 square km
Now you can convert this to decimal
I hope this helps