The given expression is:

$5 \left[ 24 - \left(9 + 3\right) \div 4 \right]$

Let's solve this step-by-step, following the order of operations (PEMDAS/BODMAS: Parentheses/Brackets, Exponents/Orders, Multiplication and Division (from left to right), Addition and Subtraction (from left to right)).

1. Inside the parentheses:
   $9 + 3 = 12$

   So, the expression becomes: $5 \left[ 24 - \left(12 \div 4\right) \right]$

2. Division inside the brackets:
   $12 \div 4 = 3$

   The expression now is: $5 \left[ 24 - 3 \right]$

3. Subtraction inside the brackets:
   $24 - 3 = 21$

   The expression simplifies to: $5 \times 21$

4. Final multiplication:
   $5 \times 21 = 105$

So, the final answer is:

$\boxed{105}$

Would you like any further details or have any questions about this solution?

Related Questions:

1. How do you solve an expression with multiple parentheses and brackets?
2. What is the order of operations, and why is it important in mathematics?
3. How do you simplify expressions involving both multiplication and addition?
4. What happens if you change the order of operations in an expression?
5. How do you handle more complex expressions involving exponents and roots?

Tip:

Always work from the innermost parentheses or brackets outwards, following the correct order of operations to avoid mistakes.