chem enthalpy change exam question

this may be a stupid question but why is the mass that is used in the formula for temperature change 100g and not 50g
mass using density is density x volume so shouldnt it be 1.00x50?

this is the question btw

In a different experiment 50.0 cm3 of 0.500 mol dm–3 aqueous hydrochloric acid are
reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3

The markscheme has this as the answer:

n(HCl) or n(NaOH) = 50 x 0.500 / 1000 = 0.025 moles
q = –ΔH x n = 57.1 x 0.025 = 1.4275 kJ
ΔT = q/mc
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
Final Temperature = 18.5 + 3.4 = 21.9 °C

When you mix together 50g of one component and 50g of the other component how much mass do you have?