How long is 9/10 inches

-- It's exactly 10% shorter than 1 inch, and 92.5% shorter than 1 foot.

-- It's also approximately 0.01343 smoot (rounded). 

-- It's also approximately 2.286 centimeters, or 22.86 millimeters,
or 0.0000142 mile.

-- It's also approximately 0.000 000 000 000 000 002 418 light-year.

You see, "9/10 inch" is known and understood precisely wherever inches are
used in the world (everywhere throughout Liberia, Burma, and the USA) exactly
because "inch" is a unit that's exactly defined and agreed on in all those places. 
But when you come along and say "I don't know what an inch is, describe it to me
in terms of something else", then you throw people for a loop.

If you can tell us what other unit you want to measure with ... like a flower,
a finger, an animal, or something else that we're also familiar with, then
we'll compare "9/10 inch" to that item for you.  But that won't be exact,
because all of those things come in different sizes, and none of them has
a standard size that everybody agrees on.

Ais the answer for your question