Add a number to the numerator and denominator of the fraction 1999 / 1997, and the new fraction is equal to 1999 / 2000?

Add a number to the numerator and denominator of the fraction 1999 / 1997, and the new fraction is equal to 1999 / 2000?

Let the number added be X
(1997+x）/（1999+x)=1999/2000
1999(1999+x)=2000（1997+x)
x=2001
The sum is 2001

The numerator and denominator of the 1991 / 1993 fraction plus a natural number is the new fraction, 1994, 1995?

Let this natural number be X
（1991+x）/（1993+x）=1994/1995
1995*（1991+x）=1994*（1993+x）
x=1994*1993-1995*1991
x=1997
This natural number is 1997

Six fractions, one-half, one-third, one-fifth, one seventh, one tenth, one thirteenth and which two continuous natural numbers
I want to use the sixth grade formula to solve the problem*^_ ^*)
Give me the answer by 12:30 on April 15