U can use the equation KCF it means Keep Change Flip.
U keep 5/6 change the operation to multiplication from division and flip the fraction 7/8 to 8/7.
5 times 8 =40 so the numerator is 40.
6 times 7 =42 so the denominator is 42.
The fraction is 40/42.
In simplest from it looks like this 20/21