In the commercial manufacture of nitric acid, how many liters of nitrogen dioxide will produce 30 liters of nitric oxide given that both gases are at stp?

3NO2 + H2O = 2HNO3 + NO

moles of nitric oxide:
under STP (Standard Tempreture and Pressure) 
22.4L of any gas = 1mole

so 30L of NO: 30 / 22.4 = 1.33928571429 moles

so you use the ratios 3:1 to find the moles of the nitrogen dioxide: 1.33928571429 x 3 (is the ratio) =4.01785714286mol⁻¹ of NO₂

now to find liters use: L = Moles x 22.4
L of NO₂ = 90L

hope that helps