Saving for Phone Case

Kevin saved up to buy a new phone case costing CZK 390. He only put ten-crown and twenty-crown coins in his money box. After two months of saving, he had 25 coins in his money box. When he calculated their total value, he found that he still needed more than CZK 50 for the new case.

Final Answer:

a =  16
b =  9

Step-by-step explanation:


a+b = 25
10a+20b + 50 = 390

a+b = 25
10·a+20·b + 50 = 390

a+b = 25
10a+20b = 340

Pivot: Row 1 ↔ Row 2
10a+20b = 340
a+b = 25

Row 2 - 1/10 · Row 1 → Row 2
10a+20b = 340
-b = -9


b = -9/-1 = 9
a = 340-20b/10 = 340-20 · 9/10 = 16

a = 16
b = 9

Our linear equations calculator calculates it.



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