Atank that is 40% full contains 648 gallons of water. use a double number line to find the maximum capacity of the water tank.

We know that
Double number lines are best used to visualize equivalent ratios and describe a ratio relationship between two quantities.

start with the par 40% - 648 gal and let's calculate the other missing data
to find 10%> multiply 648 gal by (1/4)
to find 20%> multiply 648 gal by (1/2)
to find 30%> multiply 648 gal by (3/4)
to find 50%> multiply 648 gal by (5/4)
to find 60%> multiply 648 gal by (6/4)
to find 70%> multiply 648 gal by (7/4)
to find 80%> multiply 648 gal by (8/4)
to find 90%> multiply 648 gal by (9/4)
to find 100%> multiply 648 gal by (10/4)

Here is what the numbers would look like on a number line.  They are lined up.
10%  20%  30%  40%  50%  60%  70%  80%  90%  100%
162   324   486   648   810   972   1134  1296  1458  1620

the answer is
the maximum capacity is 1620 gallons of water.