At STP (standard pressure and temperature) each mole of gas occupies a volume of 22.4 liters.
The balanced chemical reaction tells you that 3 moles of nitrogen dioxide produce 1 mol of nitric oxide (both gases as per problem specifications).
3 moles 22.4 = 67.2 liters
1 moles 22.4 liters = 22.4
Ratio: 67.2 liters of nitrogen dioxide / 22.4 liters of nitric oxide = x liters of nitrogen dioxide / 30 liters of nitric oxide
Solve for x: x = 30 liters of nitric oxyde * 67.2 liters of nitriogen dioxide / 22.4 liters of nitric oxide = 90 liters oxide.nitrogen dioxide.
