Advanced Mathematics Primary 5 Angles

Two straight lines intersect at point O. One of the angles formed is (4y + 5)° and the angle vertically opposite to it is (3y + 20)°. Find the reflex angle at O on the same side as these two angles.

A 230°
B 245°
C 255°
D 295°
Show Worked Solution

Worked Solution

Step 1: Vertically opposite angles are equal. 4y + 5 = 3y + 20 Step 2: Solve for y. 4y − 3y = 20 − 5 y = 15 Step 3: Find the acute/obtuse angle at O. Angle = (4y + 5)° = 4(15) + 5 = 60 + 5 = 65° Verify with VO: (3y + 20)° = 3(15) + 20 = 45 + 20 = 65° ✓ Step 4: Find the reflex angle. Reflex angle = 360° − 65° = 295° Answer: 295° Answer: 295°

Correct answer: 295°

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