You would use the **Ideal Gas Law:**

#PV=nRT#

All of the variables you indicate in your question pertain to this equation, except for R, which is the gas constant, and which you might need to look up.

**Organize your data:**

**Known**

#P="20000 kPa"#

#V="25 L"#

#T="300 K"#

#R="8.3144598 L kPa K"^(−1) "mol"^(−1)"#

https://en.wikipedia.org/wiki/Gas_constant

**Unknown:** moles: #n# (moles)

**Solution**

Rearrange the ideal gas law equation to isolate #n# on the left side. Insert your data and solve.

#n=(PV)/(RT)#

#n=(20000color(red)cancel(color(black)("kPa"))xx25color(red)cancel(color(black)("L")))/(8.3144598 color(red)cancel(color(black)(" L")) color(red)cancel(color(black)(" kPa")) color(red)cancel(color(black)(" K"))^(−1) "mol"^(−1)xx300color(red)cancel(color(black)("K")))="200 mol air"# rounded to one sig fig