Question 818900
Put some numbers into the description:
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Josh's Coffee Shop makes a blend that is a mixture of two types of coffee. Type A coffee costs Josh L per pound, and type B coffee costs H per pound. This month, Josh made M pounds of the blend, for a total cost of c (in whatever money units). How many pounds of type A coffee did he use?


Accounting for pounds of the coffee blend, let u = pounds of type A and let v = pounds of type B, {{{u+v=M}}};


Accounting for money or cost for the M pounds of the blend, {{{Lu+Hv=c}}}


The question asks only  how much type A coffee were used.  Try solving either equation for v, and substitute this formula into the other equation and solve it for u. .  All other variables are assumed known.  Solve this system for u and v:
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Lu+Hv=c
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u+v=M
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You could also solve for BOTH u and v if you wanted.