Question 359550
Let {{{S}}} be the cost of the shirt, {{{T}}} the trousers.
{{{6S+2T=1320}}}
{{{3S+4T=1290}}}
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[A]={{{(matrix(2,2,6,2,3,4))}}}
[x]={{{(matrix(2,1,S,T))}}}
[b]={{{(matrix(2,1,1320,1290))}}}
[A][x]=[b]
[x]=[A]inv[b]
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det[A]={{{18}}}
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[A]inv={{{(1/18)*(matrix(2,2,4,-2,-3,6))}}}
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[x]={{{(matrix(2,1,S,T))=(matrix(2,1,150,210))}}}
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So then 2 shirts and 2 trousers would be 
{{{P=2S+2T=2(150)+2(210)=300+420=highlight(720)}}}