It all depends on what imperial measurement you want to convert to metric e.g. if you want to convert 10 km into miles, this is the method:
You have to memorise the fact that 5 miles is approx 8 km.
Therefore 10 miles is double the number of km.
So, (5 x 2 = 10) 10 miles is 8 x 2 = 16 km.
Now, if you want to find out how many miles is 2.5 km (5/2 = 2.5), divide 8 by 2 = 4. So 2.5 km = approx 4 miles.