This task contains an ELF file that needs to be decompiled. The program itself asks “What’s my favorite number?” and expects some number, after which a flag will be displayed. Otherwise, it outputs “Sorry, that's not it!”.
To do this, we will use an online tool Decompiler Explorer
Here, attention is drawn to the Ghidra window, where there is an obfuscated function and an if check:
if (local_48 == 0x86187)
0x86187 in decimal gives 549255. So, let’s try 549255:
picoCTF{cu7_7h3_bl047_44f74a60}