Official Guide Explanation:
Data Sufficiency #137




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Solution and Metadata

Question: 137
Page: 285
Difficulty: 4 (Moderately Easy)
Category 1: Algebra > Linear Equations-Two Unk >

Explanation: If the price of a donut is d and the price of a bagel is b, we're looking for 5d + 3b.

Statement (1) is sufficient. It gives us an equation:

10d + 6b = 12.90

That's one equation with two variables, but notice that the ratio of 10 to 6 is the same as that of 5 to 3. Divide both sides of the equation by 2 to make it look even more similar:

5d + 3b = 6.45

There's your answer.

Statement (2) is insufficient. Another equation:

d = b - 0.15

This time, we can't manipulate the equation to find the value of 5d + 3b. We could substitute b - 0.15 for d, but that would leave us with an expression with a variable remaining:

5(b - 0.15) + 3b

Choice (A) is correct.

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