Twenty-five 81116

Twenty-five pencils were bought as prizes in the school competition. The more expensive pencils were for 20 CZK, the cheaper ones for 15 CZK. The entire amount paid was 455 CZK. How many are there?

Correct answer:

a =  16
b =  9

Step-by-step explanation:


a+b=25
20a+15b=455

a+b=25
20·a+15·b=455

a+b = 25
20a+15b = 455

Pivot: Row 1 ↔ Row 2
20a+15b = 455
a+b = 25

Row 2 - 1/20 · Row 1 → Row 2
20a+15b = 455
0.25b = 2.25


b = 2.25/0.25 = 9
a = 455-15b/20 = 455-15 · 9/20 = 16

a = 16
b = 9

Our linear equations calculator calculates it.



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