[Math] How many $3$-digit numbers larger than $700$ can be formed by using the digits $1$, $5$, $7$, $8$, and $9$ without repetition

Question

Given the numbers $1$, $5$, $7$, $8$, and $9$, how many $3$-digit numbers larger than $700$ can be formed if repetition is not allowed?

The answer is $36$.

I want a detailed explanation please of how we get this answer?

Answer 1

Let your number be $ABC$,
$A $ can take up values 7,8 and 9 i.e. total 3. Now $B$ has total 4 choices and simultaneously $C$ will have 3.
Thus answer=4*3*3=36

Example: Let $A$=7. Thus $B$ is left with {1,5,8,9}.
Let $B$ be 5. Thus now $C$ is left with {1,8,9}. Clear?