20/80% = Lo/100%
Lo = 20*100/80 = 25 cm
K = F/Δx = 75*100/(25-20) = 75*20 = 1.500 N/m (1,5*10^3 N/m)
x'=x-20%x=20cm; x'=(1-0,2)x x=20/0,8=25 cm
poi dalla legge du hooke trovi K
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Lunghezza della molla a riposo $\small l_0= \dfrac{l_1}{\left(1-\dfrac{20}{100}\right)}= \dfrac{20}{0,8} = 25\,cm;$
compressione $\small (\Delta{x})$ $\small \delta= l_0-l_1= 25-20 = 5\,cm;$
costante elastica $\small k= \dfrac{F\,(N)}{\delta\,(m)} = \dfrac{75}{5×10^{-2}}= \dfrac{75}{0,05} = 1500\,N/m\;(= 1,5×10^3\,N/m).$