The sum of three numbers A, B, and C is 300. It is known that A is 50 greater than B, and B is 20 greater than C. What is the number of A?
甲、乙、丙三个数，和为300，已知甲比乙大50，乙比丙大20，甲数是多少？
<gadget id="calculator">300 - 50</gadget>
<output>250</output>

<gadget id="calculator">20 + 250</gadget>
<output>270</output>

<gadget id="calculator">1 + 1 + 1</gadget>
<output>3</output>

<gadget id="calculator">270 / 3</gadget>
<output>90</output>

<gadget id="calculator">90 + 50</gadget>
<output>140</output>

<result>140</result>