this is a combined gas law problem. the general formula for this is

 P1V1 / T1 = P2V2 / T2 to get the equation for this problem just take out the unknown then do cross multiplication of the factors the one with most factors will be your dividend and the one with lesser factors will be the divisor, so the equation for this is

V2 = P1V1T2 /P2Ti  then proceed to substitution

V2 = 700mmHg X 600 ml X 253.15 / 500mmHg X 300.15

      = 106,323,00 / 150,075

  V2    =708.465 ml is the final answer

Note to get temperature in kelvin for T1=  27 + 273.15 = 300.15k

                                                           T2= 273.15 + -20 = 253.15

* for the unit it will be ml since we cancelled mmHg and K because both dividend and divisor has the unit only ml is not cancelled