From the graph, we can note that the points are in a line.
Hence, we must find the line equation for these points.
The general form of the straigh line equation is

where m is the slope and b the y-intercept. The slope can be computed as

where,

By substituying these values into m, we have

hence,

the form of the line equation is

where x is the blue paint and y the white paint.
In order to find b, we can substitute one point into the above equation. For instance, the point
(2,4):

Thefore, the line equation is

Hence, the number of galons when we mix 1 gallon of blue pain is

in other words, for 1 gallon of blue paint we must have 2 gallons of white paint