x/r + x/w + x/t = 1
x (1/r + 1/w + 1/t) = 1
x = 1 / (1/r + 1/w + 1/t)
To make the solution a bit cleaner, multiply through the numerator and denominator by the LCM of each fraction's denominator, rwt :
x = 1 / (1/r + 1/w + 1/t) • rwt / rwt
x = rwt / (rwt/r + rwt/w + rwt/t)
x = rwt / (wt + rt + rw)
