Question 1018567
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3u+z=15
U+2z=10
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3u +  z = 15,    (1)
 u + 2z = 10.    (2)

Substitute u = 10 - 2z from (2) into (1). You will get a single equation for u:

3*(10-2z) + 2z = 10.

Simplify and solve it:

30 - 6z + 2z = 10,

30 - 10 = 4z, 

4z = 20, 

z = {{{20/4}}} = 5.

Now find u from (2).
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