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kevincan
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GRE 1: Q170 V170
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Great follow-up question, achyut123 — this tripped me up early in my prep too!

In Case B, the equation is 9a + 4c = 68. Since both a (adult tickets) and c (children's tickets) must be non-negative integers, you can't just pick random values — you're constrained by what actually works.

The trick is to test values of a that make 4c an integer: c = (68 − 9a)/4. For c to be a whole number, (68 − 9a) must be divisible by 4. Since 68 is divisible by 4, you need 9a divisible by 4 too — which means a must be divisible by 4. So the only candidates are a = 0, 4, 8, 12... and you stop when c goes negative.
- a = 0: c = 17 (total = 17)
- a = 4: c = 8 (total = 12)
- a = 8: c = -1 (invalid)

So there's no need to test every value of a — the divisibility filter narrows it down immediately. This "divisibility filter" shortcut is one of the most useful Data Sufficiency techniques for integer constraint problems.

You'll see this pattern a lot in DS — keep it in your toolkit!

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Kavya | 725 GMAT Focus | Free gamified GMAT prep: edskore.com
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