Name one advantage of the metric system over the customary system.

By "customary system", do you mean the English system aka the imperial system?

here are some examples.

1. see attachment
metric is all base 10.
1m=100cm=1000mm
measurements like kg and g and mg are sized by prefexes that are powers of 10

in imperial, we've got 12in=1ft and 3ft=1yard and other rediculous stuff like that

a comparison from tumblr is below

"to remember how many feet there are in a mile, u just gotta use 5 tomatoes

five to-mate-oes sounds like five, two, eight, 0 and there’s 5280 feet in a mile

To remember how many meters there are in a kilometre you just remember “1000” because the system of measurement in the rest of the world wasn’t invented by a drunk mathematician rolling dice."

2. metric has a separate measurement for Mass and force (kg and Newton) while English has. lbs being used for both (technically mass is slugs and force is lbs but who uses slugs to find mass?

I conclude with this exerpt from Josh Bazell's Wild Thing
“In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade—which is 1 percent of the difference between its freezing point and its boiling point. An amount of hydrogen weighing the same amount has exactly one mole of atoms in it. Whereas in the American system, the answer to ‘How much energy does it take to boil a room-temperature gallon of water?’ is ‘***********,’ because you can’t directly relate any of those quantities.” 

221,500 is the answer