The combinatorics of the colliding bullets problem

The finite colliding bullets problem is the following simple problem: consider a gun, whose barrel remains in a fixed direction; let $(V_i)_{1\le i\le n}$beani\mathrm{.}i\mathrm{.}d\mathrm{.}familyofrandomvariableswithuniformdistributionon$[0,1]$;shoot$n$bulletsoneafteranotherattimes$1,2,\dots, n$,wherethe$i$thbullethasspeed$V_i$. When two bullets collide, they both annihilate. We give the distribution of the number of surviving bullets, and in some generalisation of this model. While the distribution is relatively simple (and we found a number of bold claims online), our proof is surprisingly intricate and mixes combinatorial and geometric arguments; we argue that any rigorous argument must very likely be rather elaborate.