Assuming ideal behavior of the gas for a fixed amount when temperature is held constant, the pressure and volume are inversely proportional as given by the expression
P1V1 = P2V2
where the terms with subscripts of one represent the initial conditions for pressure and volume of the gas while for terms with subscripts of two represent the final conditions.
Rearranging the Boyle's law equation to calculate for the final volume V2:
V2 = P1V1 / P2
V2 = (99.7 kPa)(150 mL) / 99.8 kPa
V2 = 149.85 mL
