[ibstudent]

Hi!

Attached is a question I really don't understand which involves the rearrangement of the ideal gas law equation... the mark scheme is also attached...

As far as I know, Mr is not included in the ideal gas law equation, so where did it come from?

If anyone could clear my doubt, I will very much appreciate it 

[UG]

They substituted 'n' for 'm/M'

[Nisarg Shah]

As far as I know, Mr is not included in the ideal gas law equation, so where did it come from?

PV=nRT
Since n is number of moles, which is defined as given mass/molecular mass.
PV=(m/M)RT, where m stands for given mass and M stands for molecular mass.

[billnotgatez]

I think the original question is what is the meaning of
M_r

M sub r
rather than just
M

[Borek]

M_r is just another symbol for molar mass.