I haven't done much math since high school, so please be patient with me.

100 * 0.8 = 80

80 * X = 100

X is equal to 1.25, which was pretty easy to find after trial and error, but the trial and error process takes a lot longer with something like this:

5,680 * 0.802 = 4555.36

4555.36 * X = 5,680

X is equal to ~1.2468827930174563591022444, but that took forever to find through trial and error. How can I easily find X?