Answer
Number of investors having only IBM shares is $15$.
Work Step by Step
There are three stocks IBM, AT&T and GE. This can be presented in a Venn Diagram where I represent IBM, A represents, AT&T and G represents GE.
There are 5 investors having shares in all the companies, 20 investors had shares in IBM and AT&T. 15 investors have shares in GE and AT&T, and 20 investors have shares in GE and IBM.
In total, 45 investors had GE shares, 40 investors had AT&T shares, and 50 investors had IBM shares.
I = 50
G = 45
A = 40
A∩G = 15
G∩I = 20
A∩I = 20
A∩G∩I = 5
Then,
A∩G only = A∩G - A∩G∩I = 15 – 5 = 10
G∩I only = G∩I - A∩G∩I = 20 – 5 = 15
A∩I only = A∩I - A∩G∩I = 20 – 5 = 15
Then,
Number of investors having only GE shares, G only = G - G∩I only - A∩G only - A∩G∩I
= 45 – 15 – 10 – 5
= 15
Number of investors having only AT&T shares, A only = A - A∩I only - A∩G only - A∩G∩I
= 40 – 15 – 10 – 5
= 10
Number of investors having only IBM shares, I only = I - G∩I only - A∩I only - A∩G∩I
= 50 – 15 – 15 – 5
= 15
