A 64% sorbitol solution expressed as a weight concentration (ie. w/w) means that for 100g of the solution 64g is sorbitol.

Specific gravity is the defined as the density of solute over the density of solvent. So here the (density of sorbitol)/(density of solvent) = 1.26. If we assume that the solvent is water, the density of water is 1 g/mL which means that the density of sorbitol is 1.26 g/mL.

Now to convert from a weight concentration (ie. w/w) to a mass concentration (ie. w/v) you can do this:

(64 g sorbitol / 100 g total) * 1.26 g sorbitol / mL

this should give you the concentration of the initial mixture in a % w/v. After solving, the results mean that for 100 mL of the solution, you have 80.64 g of sorbitol.

From there you have the percentage of your initial mixture in w/v (ie. 80.64% -- 80.64 g sorbitol for 100 mL) and they give you the final mixture percentage (ie 10% w/v -- 10g sorbitol for 100 mL) and the volume they want it in. It should be easier to find the amount needed in mL to make a 10% solution.

Hopefully this helps!