Answer:
See below.
Explanation:
Call the three unknown values x, y, and z, as shown in the picture below.
According to the rule that you add two numbers to get the number below, you end up with these three equations:
x + 4 = y
x + 9 = z
y + z = 35
Now we have a system of 3 equations in 3 unknowns.
Add the first two equations:
2x + 13 = y + z
The third equation is y + z = 35, so substitute 35 for y + z in the equation above and solve for x.
2x + 13 = 35
2x = 22
x = 11
y = x + 4 = 11 + 4 = 15
z = x + 9 = 11 + 9 = 20
Now fill in the grid. See the second picture below.