The teacher will be very surprised to see your correct solution 😉
261*s 717
s - ----- = ---
500 200
s-(261/500)*s = (717/200)
s-261/500s = (717/200)
s-261/500s = 717/200
239*s/500 = 717/200
s = 717/200 / (239/500)
sum
15/2
=
15/2
product
15/2
=
15/2
15/2