This is a small app for Android that scratches my personal itch.
As a "language person" (as opposed to "number people") I have sometimes trouble doing simple math in my head. This simple app allows me to always have a fast way to do cross-multiplications.
How is this useful for you? Easy; for example to calculate what the final prize is for an item that has a 15% discount. Or to know how much 350 grams of cheese costs if you only know the prize per 100 grams. Now that's awesome, innit?