The three students whose individual number of shirt owned sums up to 20 are Will, Mandy and Cindy.
Given the information about the number of t-shirts owned by 4 students :
To obtain the which three students have shirts which sums up to 20 ;
Will + Mandy + Cindy
4 + 7 + 9 = 20 students
Therefore, the three students whosw shirts sums up to 20
are Will, Mandy and Cindy.
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