On solving x + 4y = 25 and x + y = 10, what is the value of y?

Correct answer: y = 5

The governing concept is elimination of a common variable. Since x has coefficient 1 in both equations, subtracting the second equation from the first removes x: (x + 4y) − (x + y) = 25 − 10. The result is 3y = 15, and division by 3 gives y = 5. Thus option B is correct. A complete check uses the second equation: if y = 5, then x + 5 = 10, so x = 5. Substitution into the first equation gives x + 4y = 5 + 4(5) = 5 + 20 = 25, exactly as required. The alternatives 4, 6, and 7 would give different values of x from the two equations, so none can be the common value of y for this pair.