Incorrectly 82480

Peter solves 40 examples. For a correctly solved example, he receives CZK 5 from his mother. For example, in a poorly solved case, he will pay 20 CZK to his mother. After solving all the examples, he had CZK 25 more than when he started counting. How many examples did Peter solve correctly, and how many incorrectly?

Correct answer:

a =  33
b =  7

Step-by-step explanation:


40=a+b
25 = 5a-20b

40=a+b
25 = 5·a-20·b

a+b = 40
5a-20b = 25

Pivot: Row 1 ↔ Row 2
5a-20b = 25
a+b = 40

Row 2 - 1/5 · Row 1 → Row 2
5a-20b = 25
5b = 35


b = 35/5 = 7
a = 25+20b/5 = 25+20 · 7/5 = 33

a = 33
b = 7

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