I've had this puzzle on my phone for years. Every few months I'd glance at it and try and solve it. Last night, as I was eating dinner, I finally figured out the answer!

https://sh.itjust.works/post/55641581

15, right? You just add the left and bottom to get the right part

Edit: it was not 15 ;-;

Edit: sorry

Not going to lie…I.gave it to Claude A.I. and he agrees with you
OP said not to share your solution…
You are a jerk and your spoiler did not work.
It worked on my side. I’m sorry
I wonder if my Lemmy app sucks. About half the posts with spoiler tags successfully show blank spoiler lines, but the other half, like yours, don’t work for me and just reveal the text. Weird that some work and others don’t, but I can’t assume that half these people suck at tags so much… Must be my app. Sorry for calling you a jerk.