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Please notice on how I edited your post.
what is the percent decrease? 0.75 to 0.3? step by step, .
Solution
Technically, it is very easy.
But the key moment in solution such problems is to correctly use the base value for comparison.
If the author of the problem does want to get the precise answer, he / (or she) must point this base of comparison
directly and explicitly.
If it is not pointed directly and explicitly, then the commonly accepted rule is to use the ORIGINAL value
as the base for comparison BY DEFAULT.
So I will do it in this case:
The percent of decrease is = 60%.
In wording form :
a) calculate the difference;
b) relate it to the original value;
c) multiply the ratio by 100 to get the percentage.
That's all. // Solved.