FMsensitivity analysis

hi. i used the same approach (in OP) for sensitivity qn, dec 01 tower railways inc. i got (NPV/rev)2003/10771 X 100% = 18.6% no where near their ans. of 20.87% or fall in px of $2.50. Moreover their explanations n calculations are far more confusing !! pls help !!! tks