Calculating % Contribution

[thinkpositive]

I am trying to understand how to calculate percent contribution.

In the example provided for product A, B, and C how do you determine the contribution to the 19% Month to Month change, i.e. A x% + B x% + C x% = 19%

Any help is greatly appreciated!

Product Month 1 Actual Month 1 Goal Attainment A 500 1,000 50% B 700 1,500 47% C 900 1,100 82% Total 2,100 3,600 58% Month 2 Actual Month 2 Goal Attainment A 500 600 83% B 500 900 56% C 1,700 2,000 85% Total 2,700 3,500 77% MTM +19%