Official Guide Explanation:
Data Sufficiency #21

Background

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Solution and Metadata

Question: 21
Page: 154
Difficulty: 6 (Moderately Difficult)
Category 1: Word Problems > Other >
Category 2: Arithmetic > Properties of Integers > Other

Explanation: Statement (1) is sufficient. First, figure out the sum of the units digits on the original 36 cards. Each set of 9 has a sum of 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45, meaning that the four sets of 9 have a total of 45(4) = 180, which is a units digit of 0. If the units digit of the sum of the remaining 35 cards is 6, that means that the missing card is a 4.

Statement (2) is also sufficient. It tells us the same thing that (1) did, only with some extraneous detail. Again, we can determine that the missing card is a 4. Choice (D) is correct.

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