welcome @Carolk on the ODK forum. We you get a chance please take some time to introduce yourself here
We have the same need to compare a integer value with the sum of selected value in a select_multiple displayed as an image map. In fact to compare à turtle ID to the code marked on its shell.
Will try to find a way.

@Vivian_Inereli and I found two solutions, from the forum.
The first one needs you to know the number of selected values in your select_multiple (wich is the same a space separated values) and use one calculate of selected-at per value Calculations on select multiple - #5 by Naveen_Agarwal

And the second one calculate the number of selected values, and within a repeat, iterate to get each individual value. Outside the repeat we use the sum() function. select_multiple_options_sum_with_sum_function.xlsx (15,8 Ko)