Final answer:
Emilia's expression X + 0.4x simplifies to 1.4x by factoring out the common variable X. Thus, option C, 1.4x, is equivalent to the original expression.
Step-by-step explanation:
The expression Emilia wrote, X + 0.4x, is a simple algebraic expression that combines a variable, X, and 40% of that same variable. To find another way to write this expression, we can factor out X from both terms. Doing so, we see that X is a common factor and we have:
X(1 + 0.4) = X (1.4)
This simplifies to 1.4x, as 1 X + 0.4 X equals 1.4 times X.
Therefore, among the provided options, C. 1.4x is an equivalent expression to X + 0.4x, since it maintains the same relationship between the variables and coefficients.