c)
\frac{3,72+3,54+3,30+1,90}{4}=3,115\approx3,12[zl] średnia cena w maju 2005
\frac{4.56+4,29+3,96+2,03}{4}=3,71[zl] w maju 2006
\frac{3,71-3,12}{3,12}*100\%=18,91...\%\approx19\%
d)
3,72*250+3,54*500+3,30*200+1,90*250=3835[zl] maj 2005
4.56*250+4,29*500+3,96*200+2,03*250=4584.50[zl]
\frac{4584,50-3835}{3835}*100\%=19,54...\%\approx 20\%