托马斯和西蒂拥有的饼干数量之比为 3 : 7。托马斯又买了 42 个饼干，而西蒂吃掉了她自己的 18 个饼干后，他们剩下的饼干数量相等。西蒂起初有多少个饼干？答：___

它不会直接告诉你答案，而是问你卡在哪一步看看 AI 是怎么陪你一步步弄懂这道题的

完整解法

1. Set up the initial ratio in terms of units

Let Thomas's initial cookies be $3u$ and Siti's initial cookies be $7u$ .

2. Form an equation based on the final equal amounts

Thomas's final cookies = $3u + 42$
Siti's final cookies = $7u - 18$
Since they had an equal number of cookies left:
$3u + 42 = 7u - 18$

3. Solve for 1 unit ($u$)

$7u - 3u = 42 + 18$
$4u = 60$
$u = 15$

4. Calculate Siti's initial number of cookies

Siti's initial cookies = $7u = 7 \times 15 = 105$

答案

$105$

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