13. Fill in this diagram so that each of the rows, columns and diagonals adds to 18.
What is the sum of all the corner numbers?
(A) 20
(B) 22
(C) 23
(D) 24
(E) 25

D. 24

Step-by-step explanation:

Let's name the diagram

Row 1: a1, a2, a3

Row 2: b1, b2, b3

Row 3: c1, c2, c3

From the diagram,

b2 and c2 have been given

All rows, columns and diagonal must sum up to 18

If b2=6 and c2=4

a2=18-(6+4)

=18-10

=8

a2=8

Assume a1=4

Diagonals a1+b3+c3=18

4+6+c3=18

10+c3=18

c3=18-10

=8

Assume a3=6

Diagonals a3+b2+c1=18

6+6+c1=18

12+c1=18

c1=18-12

=6

So b1=18-(6+4)

=18-10

=8

b1=8

b3=18-(6+8)

=18-14

=4

b3=4

Input all the numbers into the boxes

We have,

4 8 6

8 6 4

6 4 8

Corner numbers are a1,a3,c1,c3

=4,6,6,8

Sum of all the corner numbers=4+6+6+8

=24

D. 24

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