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[2u + 5y = 15]

Substitute (u = 5):

[2(5) + 5y = 15]

Simplify:

[10 + 5y = 15]

Subtract 10 from both sides:

[5y = 15 - 10 = 5]

Divide both sides by 5:

Solution: [u = 5, \quad y = 1]