13. If x=0.8 is the solution of 5x+4a=12.4, what is the value of a?
13、若x=0.8是5x+4a=12.4的解，则a的值为多少．
<gadget id="calculator">12.4 - 4</gadget>
<output>8.4</output>

<gadget id="calculator">8.4 / 4</gadget>
<output>2.1</output>

<result>2.1</result>