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Understanding how to properly substitute a value into an expression is a fundamental skill in algebra. In our example, we are given the expression 6.5a and told that a=4. Substitution means replacing the variable with the number it represents. Here, we see that a is a placeholder for whatever value we assign to it, in this case, 4. So, we update our expression from 6.5a to 6.5*4. This is a necessary step because it transforms the abstract algebraic expression into a concrete numerical operation that we can solve.

When substituting, always ensure that you replace variables consistently and accurately to avoid any errors in your calculations. It can be helpful to rewrite the entire expression with the substitution clearly shown, which can prevent mistakes and clarify what operation you need to perform next.

After substituting the given value into the expression, the next step is performing the operation. Operations in mathematics include addition, subtraction, multiplication, and division. In our context, we are dealing with multiplication. Once we've substituted a with 4, we end up with the arithmetic expression 6.5 * 4. This expression requires us to multiply the decimal number 6.5 by the integer 4.

Performing operations correctly is essential for correctly evaluating expressions. It's important to follow the order of operations when more than one is present, which is often remembered through the acronym PEMDAS (Parentheses, Exponents, Multiplication and Division, Addition and Subtraction). Since we only have multiplication here, we can proceed directly to multiply the two numbers.

The process of multiplying decimals is very similar to multiplying whole numbers. However, it is essential to keep track of the decimal place to ensure the correct answer. In our problem, we multiply 6.5 by 4. To do this, one might first ignore the decimal and multiply as if they were whole numbers: 65 * 4 equals 260. Now, since there is one digit to the right of the decimal in 6.5, the final answer must also have one digit to the right of the decimal. Therefore, we place the decimal to give us 26.0, or simply 26.

Remember, the number of digits to the right of the decimal point in the product must equal the total number of digits to the right of the decimals in the factors being multiplied. This rule ensures that you correctly place the decimal point in your final answer. In our example, the multiplication results in the whole number 26, which verifies our understanding and execution of decimal multiplication.