What is undefined in functions

Compare the subtraction problems 6/8 - 5/8 = 1/8 and 6/9 - 7/9 = 1/9 why is the answer to the first problem positive and the answer to the second problem negative

Jane’s favorite fruit punch consists of pear, pineapple, and plum juices in the ratio 5: 2: 3. chapter reference how much punch can she make if she has only 6 cups of plum juice?

Which has the greater energy, light of wavelength 519 nm or light with a frequency of 5.42 x 10^8 sec^-1?