The mathematical operation we must use to determine the size of an interval is division. When we divide the frequencies, both thirds have the same size or mathematical ratio (1.25):
325 / 260 = 1.25
162.5 / 130 = 1.25
This number, or mathematical ratio, allows us to calculate major thirds based on any note. By multiplying we get an ascending major third and by dividing we get a descending major third. Starting from a 440, we get the upper C# and the lower F:
Note  Frequency  Ascending major third (C#5)  Descending major third (F4) 

A4  440 Hertz 
440 * 1.25 = 550

440 / 1.25 = 352

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