let T be the total cost for all 6 chairs it sold together.
from first statement
T=0.9(6S)...where S is cost of single chair.

from this there is not way we can get the cost of single chair.

5S+20=T from statement 2.(linear equation of 2 variables)

Using these 2 equations we can get S=50.
hence (C)

U can just consider one variable if you like.

Let each chair cost X dollars.
6 cosst 6X.
the set of 6 is 10% cheaper - 5.4X

Statement 1 in sufficient.

Statement 2 : set of 6 = cost of 5 chairs + 20
=> 5x+20
Insufficient.

combine 1 and 2 together: 5.4X - 5x+20
x = 20/0.4 = 50 .

Hence C.