You got two tasks. the first task has $n_1$ solutions, the second task has $n_2$ solutions. Then the total amount of solutions between them is $n_1n_2$
You got two tasks. the first task has $n_1$ solutions, the second task has $n_2$ solutions. Then the total amount of solutions between them is $n_1 + n_2$
Definition: Permutations are a set of distinct objects in a ordered arrangement such that
$P(n, r) = \frac{n!}{(n - r)!}$
r = permutations n = amount of solutions P = amount of permutations
Note: $n! = n(n - 1)(n - 2 ) … (1)$
Definition: r-combination of elements of a set is an unordered subset of the given set denoted by $C(n, r)$ such that
$C(n, r) = n! \ r! (n -r)!$
Note: $C(n, r) = C(n, n - r)