Allowing a user enter data as a number or percentage?

[nicksoph]

Im making a model of visitors to an attraction and trying to allow users to play with the model by altering the number or percentage of people attending (classified by their age group eg, 200 people attending in total, 50 of whom are under 10, 70 who are over 10 and under 18 and 80 who are over 18 which would give percentages of 25%, 35% and 40%). The intention is to allow the user to play with either the numbers of each group - which would alter the total number of visitors and percentage of each group OR the percentage of a Group which would retain the total number of visitors but alter the percentage of each of the other groups proportionally to their current size so the proportions between the unchanged groups remains the same. I can see that I will need more than one set of data entry places and macros to copy the data between the tables but cant seem to visualise how the process would work in use. Has anybody done something similar or able to offer advice on the principles involved?

 

[mikerickson]

Be clear what you want your results to be when you change the percentages?

If the exisitng data is
25 apples 50%
15 bananas 30%
10 oranges 20%

and the user changes the 30% to 40%, what do you want the new numbers to be?

 

[nicksoph]

Be clear what you want your results to be when you change the percentages?

If the exisitng data is
25 apples 50%
15 bananas 30%
10 oranges 20%

and the user changes the 30% to 40%, what do you want the new numbers to be?

Thanks for the reply -

Changing the percentage keeps the number of fruit the same (50) and there would be
40% bananas = 20 - or 5 more bananas
So we take away 5 from the other two but retain their current ratio of 25 to 10
so 25 apples / 35 (25 apples + 10 oranges) * 5 = ~3.57 fewer apples = 21.43 apples or 42.86%
so 10 oranges / 35 (25 apples + 10 oranges) * 5 = ~1.43 fewer oranges = 8.57 oranges or 17.14%

I think I'm ok with the calculations but can't find a way to present to the user so that either changing numbers or percentage will work