A boy buys 3 apples and 1 orange juice for $\$1.35$. A girl buys an apple and 2 orange juice for $\$1.20$. how much does an apple cost?
Let $x be the cost of an apple, $y be the cost of orange juice.
\(\begin{cases}3x + y = 1.35\\x + 2y = 1.2\end{cases}\\ \begin{cases}6x + 2y = 2.7\\x + 2y = 1.2\end{cases}\\ 5x = (6x + 2y) - (x + 2y) = 2.7 - 1.2 = 1.5\\ x = 0.3\\ 0.3 + 2y = 1.2\\ y = 0.45\)
