To make a kind of parts, A makes 21 parts per hour, and B makes 13 parts per hour. To complete 199 such parts in 7 hours, how many hours must A and B work together at least?
做一种零件，甲每小时做21个，乙每小时做13个．要在7小时完成199个这种零件，甲、乙二人至少要合做多少小时．
<gadget id="calculator">21 * 7</gadget>
<output>147</output>

<gadget id="calculator">199 - 147</gadget>
<output>52</output>

<gadget id="calculator">52 / 13</gadget>
<output>4</output>

<result>4</result>