Mentally: 10% of $52 is $5.20 so 20% is twice as much or $10.40 (Answer c)
To find 10% of a number just move the decimal one place to the left and to find 20% simply double the answer you got when you found 10%.
Computational way: 20% of $52 = .2 x 52 = 10.4 which in money is $10.40