The catabolism of glucose has an equation of C6H12O6 + 6O2 = 6CO2 +6 H20. Hence for every mole of glucose, 6 moles of CO2 is produced. Given 22 grams of CO2, that is 0.5 mol CO2, we multiply this by 1/6, we get the number of moles of glucose equal to 1/12 mol. The mass of glucose needed is obtained by multiplying this by molar mass of glucose which is 180 g/mol. This is equivalent 15 grams of glucose.