SAT Math · Nonlinear equations in one variable · medium

What is the sum of all real solutions to |5x - 10| = 3x?

What is the sum of all real solutions to ∣5x−10∣=3x|5x - 10| = 3x?

Answer and explanation

Answer: A: 25/4

Split ∣5x−10∣=3x|5x - 10| = 3x into two cases. Case 1: 5x−10=3x5x - 10 = 3x gives 2x=102x = 10 and x=5x = 5. Case 2: 5x−10=−3x5x - 10 = -3x gives 8x=108x = 10, so x=54x = \frac{5}{4}. Both solutions give a nonnegative right side, so both are valid. Check x=54x = \frac{5}{4}: ∣6.25−10∣=3.75=3(1.25)|6.25 - 10| = 3.75 = 3(1.25). The sum is 5+54=2545 + \frac{5}{4} = \frac{25}{4}, choice A. Choice B (5) is just one solution. Choice C (154\frac{15}{4}) comes from 3⋅543 \cdot \frac{5}{4}. Choice D (10) comes from doubling 5. The question asks for the sum of all real solutions.

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