2.1 times 10 to the tenth power equals 21,000,000,000. 7 times 10 to the seventeenth power equals 700,000,000,000,000,000. 700,000,000,000,000,000 divided by 21,000,000,000 equals 333,333,33.3 continusly.
Where 1/3 of the 1 section had been painted, have each section have 3 parts. So each complete section will be 3/3. Where there is 4 section with 3 parts each, we have 12 parts. Thus, you painted 1/12 of the fence.