This is another example of combination system equation with overall 3 unknown values that have been very popular lately. To solve it we need simple math and logic skills. The first line usually consist of 3 same moons. So we need to divide the number at the end of the equation by 3 to find the value of the first moon. The second line include always one of the first moons and another two same moons. So we subtract the value of the first moon from the number at the end of the equation and then divide the result by 2. Now we have the value of the second moon. The third line include one moon from the second line and again two new moons, but in different order. Then again we subtract the the value of the second moon and devide the result by 2. When we have the values of all 3 moons, the last fourth line is to add them to one another by simple addition and find the missing number. This app include the following moons of other planets: Io, Moon, Europa, Callisto, Hyperion, Oberon, Titan, Ganymede and Phobos.