T-shirts

The team bought white t-shirts for CZK 100/piece and black t-shirts for CZK 80/piece. A total of 50 pieces for 4200 CZK. How many black and white T-shirts did they buy?

Correct answer:

a =  10
b =  40

Step-by-step explanation:

$$\begin{gathered} 100a+80b=4200 \\ a+b=50 \\ 100\cdot a+80\cdot b=4200 \\ a+b=50 \\ 100a+80b=4200 \\ a+b=50 \\ Row2-\frac{1}{100}\cdot Row1\to Row2 \\ 100a+80b=4200 \\ 0.2b=8 \\ b=\frac{8}{0.2}=40 \\ a=\frac{4200-80b}{100}=\frac{4200-80\cdot 40}{100}=10 \\ a=10 \\ b=40 \end{gathered}$$

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