A permutation is an arrangement of objects in a specific order. The number of permutations of n objects taken r at a time is denoted by nPr, which is equal to n!/(n-r)!, where n! (n factorial) is the product of all positive integers up to n. For example, the number of ways to arrange the letters A, B, and C in a line is 3P3 = 3!/(3-3)! = 6.

A combination is a selection of objects without regard to order. The number of combinations of n objects taken r at a time is denoted by nCr, which is equal to n!/(r!(n-r)!), where n! (n factorial) is the product of all positive integers up to n. For example, the number of ways to select 2 cards from a deck of 52 cards is 52C2 = 52!/(2!(52-2)!) = 1,326.

Permutation and Combination National Book Foundation, FBISE, Federal Board Islamabad, FSC Part -1