Mom buys 3 bags of milk powder and 4 bags of washing powder, and the total cost is 57.9 yuan. It is known that each bag of washing powder is 4.2 yuan. How much is each bag of milk powder?
妈妈买3包奶粉和4袋洗衣粉，一共用去57.9元，已知洗衣粉每袋4.2元奶粉每包多少元？
<gadget id="calculator">4.2 * 4</gadget>
<output>16.8</output>

<gadget id="calculator">57.9 - 16.8</gadget>
<output>41.1</output>

<gadget id="calculator">41.1 / 3</gadget>
<output>13.7</output>

<result>13.7</result>