Physics problem. How do I solve for m when given velocity?

An astronaut wearing a jet pack is motionless in outer space. In order to move, the astronaut turns on the jet pack which ejects some gas with a velocity of +18 m/s. The astronaut recoils with a velocity of -0.5 m/s. After the gas is ejected, the m of the astronaut and his pack is 160 kg. What is the m of the ejected gas?