Question

12 miles is approximately equal to 6 km. How many km are equal to 18 miles? How many miles are equal to 42 km? 

Answer 1

so given 

12 miles = 6 km
using unitary method

12 miles = 6 km
1 miles = 6/12  km
18 miles = 6/12 x 18 km
18 miles = 9 km

6 km = 12 miles 
6 x 7 km = 12 x 7 miles
42 km = 84 miles

Answer 2

if 12 miles =6km THEN 6 MILES= 3KM
  SO 18 miles=12miles+6miles
                       =    6+3
so 18 minles =9 km
42km=?(x)
by cross multiplication
18miles=9km
?(x)=42km
18×42÷9=84
so x =84 miles
84 miles =42km