What is your mnemonic image for the number 400, and why?
In my system 400 (ROS) is roses.
Related:
400: Rhesus (monkey)
I try to avoid plurals for numbers ending in 0.
My 400 is “roses” too (Major system) and my 40 is “rose”, whenever possible and if there’s nothing better I do this “operation” 
400 - Черная МиНа black mine
Reese’s [peanut butter cups]
Because my 40 is already rose, I didn’t want it to be roses.
If the food isn’t appropriate, the vivid orange and yellow packaging, or the “Officer Reeses” scene from Family Guy adds inspiration. But it’s rarely not appropriate.
And now I have a craving!
Same as @Mnemosis. I don’t use plurals either.
oh ye of little faith, who don’t find an instance of a thing different enough to two or more of the same thing.
Kidding.
Occam’s Rosary
40 = rose (red rose even)
but
400 = Rose (almost sounds like Ross) the Hat from the Doctor Sleep (is supercool)
For me,
40 = Horse
Hah, I use a reese’s cup as well.
400 = r-oh-s in my system
So, a single rose!
