4 servings : 2 potatoes and 4 celery sticks. Find the number of potatoes and celery sticks in 6 servings. 6/4 = 1.50 2 potatoes x 1.50 = 3 potatoes 4 celery sticks x 1.50 = 6 celery sticks 6 servings of stew has 3 potatoes and 6 celery sticks. Other way of computing for the 6 servings is getting the number of potatoes and celery sticks per serving.
There are 21 ways to choose the first person, 20 ways to choose the second, 19 ways for the third and 18 ways for the fourth so the answer is 21 * 20 * 19 * 18 = 143640.