What is the advantage of valence number?​

The presence of many unpaired electrons in the valence shell of an atom enables it to form multiple bonds with other atoms. The paired electrons present in the valence shell do not take participate in the formation of chemical bonds as per the valence bond theory.

Hey there!

An easy way to look at this is with ratios.

90:30, where it takes 90 seconds per 30 jugs
x:90, where it takes x second per 90 jugs 

Figure that if it takes 90 seconds to fill 30 one–gallon jugs, then it will take three times as long to fill 90 one–gallon jugs, since 30 times 3 is 90. 

90 seconds × 3 = 270 seconds

It will take 270 seconds, or 270/60 (4.5) minutes to fill 90 jugs. Hope this helped you out! :-)