how does it become a mixed number

If you want to write a number like  3 5/7 as a mixed number:

multiply the whole number part (3) times the denominator (7):  3 x 7 = 21

and add the numerator (5):  3 x 7 + 5  =  26

Place this over the denominator:  3 5/7  =  26/7

Another example:  5 7/8  =  5 x 8 + 7 =  47 --->  47/8