You are given the sum and the difference of the ages. This is easily solved by "elimination." Let h and c represent Hannah's and Christopher's ages, respectively.
.. c -h = 5 7/12
.. c +h = 48 11/12
Add the two equations to eliminate the variable h.
.. 2c = 53 18/12 = 54 6/12
.. c = 27 3/12
Christopher is 27 years and 3 months old.
