A FORCE OF 5N STRETCHES AN ELASTIC MATERIAL 40MM,WHAT ADDITIONAL FORCE WILL STRETCH THE MATERIAL 35MM?

Answer 1

5 = 40k
k = 1/8 N/mm
to stretch 35 would not require additional force, so I guess you meant
an additional 35mm means a total stretch of 75mm
F = kx = 75/8
75/8 - 5 = 35/8 N