30 x 2 / 5 = 12 members ordered lemon juice. So, 30 - 12 = 18 members did not order lemon juice. Since 1 / 3 of the remaining ordered mango juice, then 18 x 2 / 3 = 12 members ordered mango juice. Therefore, 18 - 12 = 6 members ordered orange juice.
This justifies what answer for what question? Q & A:
A club with 30 members ordered fruit juices. Two-fifths of them ordered lemon juice. One-third of the remaining members ordered mango juice, and the rest ordered orange juice. How many members ordered orange juice?
6