There are a couple ways to do this, but I'll go with the easier among them.
We know that the 2 potatoes increased to 20.
We want to find out what number the two multiplied by to get 20. This calls for division.
20/2 = 10
Since ratios must be multiplied by the same number, we have to multiply the 3.
3 • 10 = 30
There are 30 tomatoes.