initital volume (IV) x initial strength(IS) = final volume(FV) x final strength(FS)

Final Volume -(minus) Initial Volume equals amount of diluent to make final volume.

Question: How many milliliters of water should be added to 100 mL of 10% stock solution of sodium chloride to prepare a 0.9% solution of sodium chloride.

Answer: (10%)(100 mL) = (0.9%)(X mL), where X = 1,111 mL. To calculate the amount of diluent, subtract the initial volume from the final volume (1,111 mL-100 mL = 1,011 mL of diluent).

Where the heck are they getting 1,111 from, I honestly have no idea. This has me baffled. Are there any free online guides I could use to learn some of these formulas, because I am a bit lost with this one. I generally understand how most other formulas work, but this just has me stumped.