Czech coins

John has 540 CZK, but he only has 10 CZK coins and 20 CZK coins. We know that he has 10CZK coins, 7-times more than 20CZK coins. How many 10CZK coins and how many 20CZK coins did he save?

Correct answer:

a =  42
b =  6

Step-by-step explanation:


10a+20b=540
a=7b

10·a+20·b=540
a=7·b

10a+20b = 540
a-7b = 0

Row 2 - 1/10 · Row 1 → Row 2
10a+20b = 540
-9b = -54


b = -54/-9 = 6
a = 540-20b/10 = 540-20 · 6/10 = 42

a = 42
b = 6

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