.net framework 3.5 installing problem

Jun 20, 2018
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Hii i am having a problem in installing a software...
actually when i try to install that software it says - "an app on your PC needs the following windows features :
.net framework 3.5"

i clicked on install it now through internet...
but it won't work at all..
Please help me on installing that softare..

I have windows 8.1 64 bit

(and i know that .net framework is installed in it more that the version 3.5)