You must understand the meaning of this question before you can get the answer through hands-on verification:

Take 24,2+4 = 6,6× 3+1=19; 1+9= 10, 10× 3+ 1=3 1;

3+ 1=4,4 ×3+ 1= 13; 1+3=4,4 ×3+ 1= 13 ...

Try again with 527,9, and you will find that this number is 13.