At constant temperature. a gas is at a pressure of 540 mm of mercury. At what pressure its volume will decrease by 25% ?

Dear student, 

Ideal gas equation : P V = m RT
At constant pressure, we can apply Boyle law,
P1 V1 = P2 V2
​V2 is 25% less than V1 i.e. V2 = V1* 75/100
P1 = 540mm
We can write 540* V1 = P2* 0.75V1
P2 = 540/0.75 = 720mm

Regards.