You're right about percentages being equivalent to decimals; they're also equivalent to fractions, and sometimes that's an easier way to think about them.
You can take a percentage and make a fraction out of it by putting it over 1. I'll do that with $125\%$:
$\frac{125\%}{1} = \frac{1.25}{1}$
But a fraction where one of the numbers has decimal points is kinda hard to work with, so let's multiply this by something that 1) gets rid of the decimal places and 2) is equal to 1, meaning I'll have to multiply it by a fraction that can reduce to 1....meaning the numerator and denominator have to be the same. I'll multiply it by the fraction $\frac{4}{4}$:
$\frac{1.25}{1} * \frac{4}{4} = \frac{5}{4}$
What this means is that "$125\%$ of something" is the same as "$5/4$ths of something." (I can say that they're the same because the only thing I did was to multiply it by something that's equal to $1$, so that didn't change anything except how it looks.)
So let's see what $5/4$ths of $75$ is. We'll split $75$ up into four pieces, then see what five of those pieces would add up to:
$\frac{75}{4} = 18.75$ <---So that's one fourth of 75.
Now let's see what five of those pieces comes to:
$18.75 * 5 = 93.75$
So there you go: $5/4$ths of $75$ is $93.75$. And since $\frac{5}{4}$ is just the 'fraction' way of writing $125\%$, that means that $125\%$ of $75$ is $93.75$.