Asample of gas has an initial volume of 25 l and an initial pressure of 3.5 pa. if the pressure changes to 1.3 pa, find the new volume, assuming that the temperature remains constant.

Since you are provided with the pressure and initial volume and told a standard temperature, you should use the Combined gas law. This is (P1 V1)/ T1 = (P2 V2)/ T2 
STP is 273 k for temperature, so plug in the values. 
(3.5 Pa * 25 L)/ 273 K =1.3 Pa * X L)/ 273 K.
(this is the left side>)0.32051= 1.3 X/ 273.  Multiply both sides by 273 to get rid of it on the right, which leaves you with
87.4999 = 1.3 X. Divide both sides by 1.3 and there you go, 

67.308 L