if it takes a tub 3 minutes to drain 6 / 7 of its content , how much more time will it take for the tub to be empty ?
"if 6 / 7 of tub ' s content is drained 1 / 7 th of tub still needs to be drained . if it takes 3 minutes to drain 6 / 7 th of tub it takes 3 * ( 7 / 6 ) minutes to drain the entire tub and 3 * ( 7 / 6 ) * ( 1 / 7 ) min to drain 1 / 7 th of the tub which is 6 / 5 minutes or 0.5 minutes or 30 seconds so answer is a"
add(subtract(const_1, divide(6, 7)), divide(6, 7))