1 molucule of NO₂, have 1 atom of N, and 2 atoms of O, therefore, 1 molecule of NO₂ have 3 atoms.
1 molecule of NO₂--------------------3 atoms
1.3 10²² molecules----------------- x
x=(1.3 10²² molecules 3 atoms) / 1 molecule of NO₂=3.9 10²² atoms.
Answer: in 1.3 10²² molecules of NO₂ there are 3.9 10²² atoms.