Looks to me like the numbers 1 to 19 = Monday. The numbers 20 to 39 = Tuesday, the numbers 40 to 59 = Wednesday, the numbers 60 to 79= Thursday, and the numbers 80 to 99= Friday. That might help a bit.
That's what the setter has done. He has chosen, as an example, one number from each group. Using the numbers as I have stated is the only way to find the answer.