Question 1069193
p, how much peat moss to add
v, how much vermiculite to add


PEAT MOSS - WANT 15%

{{{(p+0.12*900)/(p+v+900)=0.15}}}



VERMICULITE - WANT 15%

{{{(v+0.06*900)/(p+v+900)=0.15}}}



Those are the basic equations according to the description.  Simplify each and then solve the system for p and v.