I am not good at doing these pie questions so someone give answer and explain it in much detail as possible !!!
The thing to do is to recognise that the full membership represents 360 degrees of the pie.
So the proportion of Juniors members is 45/360 = 1/8
Now you are told the total number of members in 2000 was 800. So the number of junior members is
1/8 * 800 = 100.
Hope this helps.