please help!! I have no idea how to do these I am trying to help my son..

Find two mixed numbers so that the sum is 21 1/6 and the difference is 4 3/6?

fourth grader? I would multiply all by six, find the two numbers, then divide by six.

What two numbers have a sum of 6x(21 1/6)=127, and a difference of 6x(4 3/6)=27

Ok, start with half way between the sum 63, and then high is 13 above 63, and low is 14 below or
High: 76 Low:49
sum is 125 (low), and difference is 27
so keep the difference, but add one to each...
High 77 Low 50 sum=127, difference 27 BINGO.

now divide by six
high 77/6 Low=50/6

High 12 5/6 low 8 2/6
check sum: 21 1/6
difference : 4 3/6