1) The original formula = 2/(2 * 3)+2/(3 * 4)+2/(4 * 5)+…+2/(20 * 21) = 2 * (1/2-65438.

2) let a x kg and b y kg.

x+y= 1 15

x/5+y/4=26

x=55

y=60

I can get the answer.

Note: If you ask questions in the future, such as the score of the first question, please pay attention to the brackets, otherwise it will be ambiguous and not conducive to others' answers.