The number of cards left is 600-280=320.
So the 320 cards left are what percent? we count it like this:

=53.3..%
so the answer is 53.3%
Answer:

STEP BY STEP EXPLANATION

To make
a perfect square we should add 
Scale factor = new length/old length
scale factor = 10/2.5 = 4
The scale factor is 4.
C is the answer.........................
Step 1: Simplify 0.02 x 0.789 to 0.01578
0.01578 ÷ 3.0999
Step 2: Simplify 0.01578 ÷ 3.0999 to 0.0051
0.0051
Done! :) Decimal Form Would Be 0.0051