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Algebra Difficulty 6.4 National olympiad Find the answer

Petya bought one cake, two cupcakes and three bagels, Apya bought three cakes and a bagel, and Kolya bought six cupcakes. They all paid the same amount of money for purchases. Lena bought two cakes and two bagels. And how many cupcakes could be bought for the same amount spent to her?

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve this problem, we need to determine how many cupcakes can be purchased for the same amount that Lena spent, given the prices of each pastry type.

Let's denote the prices:
- The price of one cake as c c .
- The price of one cupcake as p p .
- The price of one bagel as b b .

According to the problem, Petya, Apya, and Kolya all spent the same amount of money, leading to the following equations:
- Petya: c+2p+3b c + 2p + 3b
- Apya: 3c+b 3c + b
- Kolya: 6p 6p

These equations imply:
c+2p+3b=3c+b=6p c + 2p + 3b = 3c + b = 6p

We want to find the number of cupcakes n n Lena could buy with the same amount of money she spent on 2 cakes and 2 bagels:
- Lena: 2c+2b 2c + 2b

First, set the expressions for Petya and Apya equal:
c+2p+3b=3c+b c + 2p + 3b = 3c + b
Simplifying, we have:
2p+2b=2c 2p + 2b = 2c
p+b=c p + b = c

Next, equate the expressions for Petya and Kolya:
c+2p+3b=6p c + 2p + 3b = 6p
Simplifying, we find:
c+3b=4p c + 3b = 4p

We now express 2c+2b 2c + 2b in terms of p p . Using p+b=c p + b = c , substitute c=p+b c = p + b into expression for Lena:
2(p+b)+2b=2p+2b+2b=2p+4b 2(p + b) + 2b = 2p + 2b + 2b = 2p + 4b

Now, relate 2p+4b 2p + 4b to 6p 6p . From c+3b=4p c + 3b = 4p , substitute c=p+b c = p + b to get:
p+b+3b=4pp+4b=3p2b=2ppp=2b p + b + 3b = 4p \quad \Rightarrow \quad p + 4b = 3p \quad \Rightarrow \quad 2b = 2p - p \quad \Rightarrow \quad p = 2b

Substitute p=2b p = 2b in Lena's equation:
2p+4b=4b+4b=8b 2p + 4b = 4b + 4b = 8b

Finally, equating 8b 8b to the equated amounts 6p 6p , notice:
8b=6p8b=6(2b)p=43b 8b = 6p \quad \Rightarrow \quad 8b = 6(2b) \quad \Rightarrow \quad p = \frac{4}{3}b

Thus, Lena could purchase 134\frac{13}{4} cupcakes for the amount she spent. Therefore, the number of cupcakes Lena could buy is:
134 \boxed{\frac{13}{4}}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.