To convert the scientific notation $7.5 \times 10^{-4}$ into a standard decimal number, the result is $0.00075$.

When you encounter a number in scientific notation, such as $7.5 \times 10^{-4}$, the notation indicates that you need to shift the decimal point. The exponent of $10$ specifies how many places to move the decimal point. A negative exponent indicates that the decimal should be moved to the left.

Start with the number $7.5$. Since the exponent is $-4$, you will move the decimal point four places to the left. To facilitate this, you can add zeros as placeholders.

1. Moving the decimal one place to the left gives you $0.75$.
2. Moving it two places to the left results in $0.075$.
3. Moving it three places to the left results in $0.0075$.
4. Finally, moving it four places to the left gives you $0.00075$.

Thus, $7.5 \times 10^{-4}$ is equivalent to $0.00075$ when expressed as a standard decimal number. This conversion process is crucial for understanding the size of the number, as shifting the decimal point to the left reduces the value. Mastering this skill is particularly useful in GCSE Maths, especially when working with very large or very small numbers in scientific contexts.