To solve this, let the red tube: x.
The blue tube: x+7, since it costs $7 more than the red tube.
Total amount spent: 25
x+x+7=25
Combine like terms.
2x+7=25
Subtract 7 from both sides.
2x=18
Divide both sides by 2.
x=9
The red tube costs $9.
We know that the blue tube costs $7 more than the red tube.
9+7=16
The blue tube costs $16.
I hope this helps :)