the problem can be solve using the formula:
q = m Cp T
where q is the heat added to the substance
m is the mass of the substance
Cp is the specific of the substance
T is the change in temperature of the subtance
q = m Cp T
515 = m ( 4.18 ) ( 20)
m = 515 / ( 4.18) ( 20 )
m = 6.16 g of water