10 : 20
look for the number with which both can be divided.......this is 10
so you can divide the ratio numbers with 10
1 : 2
OR YOU CAN CONVERT THE RATIO INTO FRACTION FORM.
this will become : 10 / 20
now divide these two : 1 / 2
convert this fraction to ratio ..... this will become 1:2
HOPE IT HELPS !!!