a + 1/b = 5
b + 1/c = 12
c + 1/a = 13
manipulating the first line by subtracting 1/b on both sides
a = 5 - 1/b
manipulating the 3rd line by subtracting 1/a on both sides.
c = 13 - 1/a
plugging a into this
c = 13 -1/(5 -1b)
c = 13 -5 +1/b
c= 8 + 1/b
plugging c into the 2nd line
b + 1/(8 +1/b) = 12
b + 1/8 + b = 12
2b + 1/8 = 12
devide both sides by two
b + 1/16 = 6
b = 11 + 15/16
plugging the value for b into the first line to get a
a + 1/(11 + 15/16) = 5
a + 1/11 + 16/15 = 5
a + 15/165 + 176/165 = 5
a + 191/165 = 5
a = 5 + 191/165
a = 1016/165
plugging b into the 2nd line to get c
11 + 15/16 + 1/c = 12
1/c = 1/16
multiply by c on both sides
1 = 1/16c
multiply by 16 on both sides
16 = c