The number of A is 19.78, the number of B is 4.6 larger than that of A, and the number of C is 3.1 smaller than that of B. What is the number of C?
甲数是19.78，乙数比甲数大4.6，丙数比乙数小3.1，丙数是多少？
<gadget id="calculator">4.6 - 3.1</gadget>
<output>1.5</output>

<gadget id="calculator">19.78 + 1.5</gadget>
<output>21.28</output>

<result>21.28</result>