Two dice are thrown together, one white and one black…

Step 1. Asha's pairs. The doubles $(1,1), \dots, (6,6)$ are $6$ pairs. The pairs with two different numbers are $^6C_2 = 15$. In all, $6 + 15 = 21$.

Step 2. The ordered outcomes. The white die has $6$ results and the black die has $6$. So $36$ outcomes.

Step 3. A total of 7. The ordered outcomes are $(1,6)$, $(2,5)$, $(3,4)$, $(4,3)$, $(5,2)$ and $(6,1)$, so $6$ of them. Asha's pairs give only $3$: $\{1,6\}$, $\{2,5\}$ and $\{3,4\}$.

Step 4. Why Asha's pairs are not equally likely. Each of the $15$ pairs with two different numbers comes from $2$ ordered outcomes. Each double comes from $1$. So a total of $7$ really takes $6$ of the $36$ outcomes, and Asha's pairs would make it look like $3$ of $21$.