What number is added to 0.035 to obtain 4.036?

The graph represents the gallons of water in a water tank with respect to the number of hours since it was completely filled

The party planning committee has to determine the number of tables needed for an upcoming event. if a square table can fit 8 people and a round table can fit 6 people, the equation 150 = 8x + 6y represents the number of each type of table needed for 150 people.

I’m really confused and need to learn this