AlgebraVery hardConstant from a known solution
Algebra practice question
The equation (x − a)/3 = (x + 2)/5 has the solution x = 13, where a is a constant. What is the value of a?
Answer: 4
At x = 13 the right side is 15/5 = 3, so (13 − a)/3 = 3 gives 13 − a = 9 and a = 4.
Common wrong answers
- If you answered 10
- 10 uses 13 − a = 3, forgetting to multiply by 3.
- If you answered 22
- 22 adds 9 to 13 instead of subtracting.
- If you answered -4
- −4 has the sign wrong.
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