The system of equations representing the number of Blue-ray discs and DVD's after t months are:
507 + 26t
925 - 12t
Each month, the number of DVD's sold decreases by 12, this can be modelled as:
number of DVD in opening month - (monthly decrease x number of months)
Number of DVD's in t months = 925 - 12t
Each month, the number of Blue-ray discs sold increases by 26, this can be modelled as:
number of blue-ray discs in opening month + (monthly increase x number of months)
Number of Blue-ray discs in t months = 507 + 26t
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