Hello,
When you are looking for a ratio, it is simply a fraction.
In the numerator, we will put the amount of the increase which is 10.4 - 7.80 = 2.6.
In the denominator, we will put the original price of $7.80.
This gives us the ration of 2.6/7.8 or just 0.33 if we divide.
We can say there was a 33% increase (or one-third).
I hope this helps, Good luck!
MrEquation