Czech coins

John has 540 CZK, but he only has 10 and 20 CZK coins. We know that he has 10 CZK coins, seven times more than 20 CZK 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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