What does it mean to have an apostrophe after a number? As in the phrase 75 degrees 20'39?

Hint: Numbers may be simplified by adding an apostrophe in place of the omitted number. An apostrophe and s are also used to form the plural of letters, numbers, signs, and terms corresponding to words.

Complete answer:
i) When an apostrophe is found in the word angles, it means 'angular minutes," angular seconds. For instance, "75degree 20′ 39" means "75 degrees, 20 minutes, and 39 seconds."
ii) Lengths - means'- feet, " - inches, as in: He is 5'4" tall. This will suggest that he stands 5 foot 4 inches tall. (approximately 165 cm)
iii) The prime symbol ′ is widely used to denote feet (ft), while the double prime symbol ′′ is commonly used to represent inches (in) (in). In watchmaking, the triple prime ′′′ represents a ligne (about 2.26 millimetres or 0.089 inches).
iv) Angles are also supported by primes. For arcminutes (160 degrees), the prime symbol ′ is used, and for arcseconds (160 degrees), the double prime ′′ is used. 3° 5′ 30′′ is an angular measurement of 3 degrees, 5 arcminutes, and 30 arcseconds.
v) Although the characters look similar to that of the apostrophe (or single or double quotation mark), the prime symbol has a distinct set of applications. Due to the absence of prime and double prime symbols on most keyboards, an apostrophe is often used in place of the prime (and a double quotation in place of the double prime).

Note: In mathematics, astronomy, linguistics, and music, the prime symbols ′, double prime symbol ′′, triple prime symbol ′′′′, and quadruple prime symbol ′′′′ are used to designate units and for other purposes.