How many moles of n2o3 contain 2.55 × 1024 oxygen atoms

1mol     3×6,02×10²³ oxygen atoms
x             2,55×10²⁴

x = 2,5×10²⁴ × 1mol / 3×6,02×10²³ ≈ 0,138*10 mol = 1,38mol

It really depends on the circumstance, but most likely because every penny doesn’t have the exact same mass, as some have scrapes/scratches, or are chipped. Which result in a different mass, so if you want to know the exact mass of a specific 10 penny’s, you can’t weigh one and multiply it by 10. If you do it’ll be really close, but not exact.
Hope this helps!