The number of decorative pillows did the realtor arrange on each twin bed and queen bed is 6 and 9 respectively.
let
x + y = 15
2x + y = 21
From equation (1)
x = 15 - y
2x + y = 21
2(15 - y) + y = 21
30 - 2y + y = 21
- 2y + y = 21 - 30
-y = -9
y = 9
Substitute y = 9 into
x + y = 15
x + 9 = 15
x = 15 - 9
x = 6
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