Let's say the price of the red tube is x, and the price of the blue tube is x + 7. Then the total cost 25 is the price of the red tube plus that of the blue tube, or x + x + 7.
[tex]2x+7=25 \\ 2x =18 \\ x=9[/tex]
The red tube cost £9, and the blue tube cost £16.
