Mark has, in his kitchen, the ingredients needed to prepare 78 different recipes with two ingredients

each. Of those 78 different recipes, there are no two recipes that share the exact same two ingredients.

How many ingredients does he have?

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Answer:

13 ingredients

Step-by-step explanation:

The number of different recipes that can be made from 2 ingredients = 78

Since there are no two recipes that share the exact same two ingredients ;

The number of recipes in Mark's kitchen is:

nCk = 78

k = Number of ingredients per recipe = 2

n = total number of ingredients in kitchen

Total recipes made = 78

n! / (n-r)!r! = 78

n! / (n - 2)!2! = 78

n(n-1)(n-2)! / (n-2)! * 2 * 1 = 78

n(n - 1) / 2 = 78

n(n-1) = 156

n² - n = 156

n² - n - 156 = 0

Factorize :

n² - 13n + 12n - 156 = 0

n(n - 13) +12(n-13) = 0

(n-13) = 0 ; (n+12) = 0

n = 13 or n = - 12

n cannot be negative

Hence, n = 13

Number of ingredients is 13

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