Greetings from Brasil...
In this type of problem, we will use Set Theory
n(D ∪ S) = n(D) + n(S) - n(D ∩ S)
as n(U) - n(D ∪ S) = 10, then
n(D ∪ S) = 70, so
n(D ∩ S) = 5 (read both)
n(D) - n(D ∩ S) = 40 (read only D)
n(S) - n(D ∩ S) = 25 (read only S)
see attached figure
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something similar
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