First of all, you should know: 1 = 60' 1' = 60'', which is easier to calculate. After adding up, the second place will be divided into one place when it is over 60, with a score of over 60.

If you subtract one more place, it's almost the same. When a person is not enough, he is the previous one.

68 2 1′ 18″+32 32′28″=(68 +32 )+(2 1'+32')+( 18''+28'')= 100 53'46''

89 -23 12′36″=88 59'60''-23 12′36″=(88 -23 )+(59'- 12')+(60''-36'')=65 47'24''