Question

A tank attached to an air compressor contains 20.0 litre of air at a temperature of 30¡ãc and guage pressure of 4.00¡Á10^5 pa. What is the mass of air, and what volume would it occupy at normal atmospheric pressure and 0¡ãc. Air is a mixture of gases, consisting of about 78% nitrogen and 21% oxygen, with small percentage of other gases. The average molecular mass is M= 28.8g/mol.
Pls, I need help I keep getting mass to be 0.117 instead of 0.115 and affects the volume 0.941 instead of 0.874.