Book Maths Puzzle Algebra

Determinant of the matrix
1 2 -1 -2
1 2 1 2
1 0 0 1
0 1 1 0

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Book Other Question

How do I solve mixture problems for example:
A 50-gallon barrel of milk is 6% butterfat. How much skim milk (no butterfat) should be mixed to make that is 3% butterfat? using a graph 7 1/10 subtracted from 4 2/5