Question 977836
p, how much of PurePets
n, how much of NewTone


{{{n+p=25}}} accounts for masses.


Accounting for vegetable:
{{{(0.35p+0.42n)/25=0.5}}}
or
{{{(35p+42n)/25=50}}}


Accounting for meat:
{{{(65p+58n)/25=50}}}


Only two equations are needed for the system because only two unknown variables need resolution.  A full system is 


{{{system(35p+42n=25*50,65p+58n=25*50,n+p=25)}}}


and then


{{{system(35p+42n=1250,65p+58n=1250,n+p=25)}}}
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Best would be to omit either of the percentage equations, and use the mass equation to substitute for either n or p.