The Variable WEIGHTED AVERAGE and SUMPRODUCT PUZZLE

1. Col A has predefined serial number and COL B has pre defined values( say prices). 2. Whenever I set a new start point ( in D2) and end point( in E2) ,what formula can be used to calculate the new Wtavg in cell D3( for wtAvg in cells in Col B.... The main question is how would they get selected for the formula like in B2 ?) ?. Please try variating D2, and E2 ,just to make sure that it works ! Please see the attached file. Thanks