Missing numbers in arithmetic sum
Brachylog, 22 16 bytes

{Ṣ∧Ị∋|}ᵐ²ịᵐ.k+~t


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-6 bytes thanks to @Fatelize

Explanation

{Ṣ∧Ị∋|}ᵐ²ịᵐ.k+~t
{     }ᵐ²                   #   for each letter in each string
 Ṣ∧Ị∋                       #       if " " return a digit; else input
     |                      #
         ịᵐ                 #   cast each string to number
            k+              #   the sum of all but the last one
              ~t            #       is equal to the last one
           .                #   output that list