WAEC 2012 Mathematics · Question 17

If x + 0.4y = 3 and y = $\frac{1}{2}$ x, find the value of (x + y)

Options

A.1 $\frac{1}{4}$
B.2 $\frac{1}{2}$
C.3 $\frac{3}{4}$
D.5

Answer: C. 3 $\frac{3}{4}$

Explanation written with AI and checked against the answer key

Analyze the question: We have two equations in x and y, and we need x + y. Concept: Use substitution. Put y = ½x into the first equation, solve for x, then find y and add. Solution: Step 1: Substitute y = ½x into x + 0.4y = 3: x + 0.4(½x) = 3 x + 0.2x = 3 1.2x = 3 x = 3 ÷ 1.2 = 2.5 Step 2: Find y: y = ½(2.5) = 1.25 Step 3: Find x + y: x + y = 2.5 + 1.25 = 3.75 = 3¾ Check: x + 0.4y = 2.5 + 0.4(1.25) = 2.5 + 0.5 = 3 ✓ Answer: x + y = 3¾, which is option C. Tip: When one variable is given in terms of the other, substitute straight away. Also, x + y = x + ½x = 1.5x, so once you know x = 2.5, you get 1.5 × 2.5 = 3.75 quickly.

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If x + 0.4y = 3 and y = 12\frac{1}{2}21​x, find the value of (x + y)

Options

Answer: C. 334\frac{3}{4}43​

If x + 0.4y = 3 and y = $\frac{1}{2}$ x, find the value of (x + y)

Answer: C. 3 $\frac{3}{4}$