Answer
a). $(2)$ decreased,
b). $166.5^{\circ}C$
Work Step by Step
a). (2) decreased
Since, $PV=nRT$, in constant volume thermometer if pressure decreases then temperature will also decrease.
b). P=1000Pa, T=$20^{\circ}C=293K$
Final pressure = 1500Pa.
Pressure increased 1.5 times so temperature will also increase 1.5 times.
Final temperature = $1.5\times 293=439.5K=166.5^{\circ}C$
