Answer :

dln
The term "three fifths" is equivalent to [tex] \frac{3}{5} [/tex].
If [tex] \frac{3}{5} [/tex] of the cast is singing, then we just have to find the rest of the fraction.
[tex]1- \frac{3}{5} \\ \frac{5}{5}- \frac{3}{5} \\ \frac{2}{5} [/tex]
well, 3/5 of the cast is singing and the whole group would be 5/5 so then that makes 5/5 minus 3/5 which equals 2/5 don't have to sing!