First, let's make 1/2 and 3/7 have the same denominator to make it easier to see. We know that there Least Common Multiple (LCM) is 14.
1/2=7/14
3/7=6/14
Wait a second... 3/7 is less than 1/2. This means that Winnie's statement is false because it is impossible that after adding more flour, she actually has less flour than what she started with.
Hope this helps! :)