**Step 1. Calculate the masses of #"Na"_2"CO"_3# and of water in the solution**

#"Mass of soln" = 47 color(red)(cancel(color(black)("mL solution"))) × "1.08 g solution"/(1 color(red)(cancel(color(black)("mL solution")))) = "50.8 g solution"#

#"Mass of Na"_2"CO"_3 = 50.8 color(red)(cancel(color(black)("g solution"))) × ("25 g Na"_2"CO"_3)/(100 color(red)(cancel(color(black)("g solution")))) = "12.7 g Na"_2"CO"_3#

**Step 2 Calculate the mass of #"CaCl"_2·6"H"_2"O"# to be added**

Let #x = "mass of CaCl"_2·6"H"_2"O"#. Then

#"Total mass" = (47 + x)color(white)(l)"g"#

#12.7/(47 + x) = 0.10#

#12.7 = 0.10(47 + x) = 4.7 + 0.10x#

#0.10x = 8.0#

#x = 8.0/0.10 = "80 g"#

You must add 80 g of #"CaCl"_2·6"H"_2"O"# to the solution.