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
