Adress marked $52.00 has a discount of 20%. how much is the discount?
a. $13.00
b. $41.60
c. $10.40
d. $12.00

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

The method to find the female dormitory is 2-d=800