Given,

Objective function is: Z = 3x + 4y

Constraints are:

2x + 2y ≤ 80

2x + 4y ≤ 120

First convert the given inequations into corresponding equations and plot them:

2x + 2y ≤ 80 → 2x + 2y = 80 (corresponding equation)

Two coordinates required to plot the equation are obtained as:

Put, x = 0 ⇒ y = 40 (0,40) - - - - first coordinate.

Put, y = 0 ⇒ x = 40 (40,0) - - - - second coordinate

Join them to get the line.

As we know, Linear inequation will be a region in a plane, and we observe that the equation divides the XY plane into 2 halves only, so we need to check which region represents the given inequation,

If the given line does not pass through origin then just put (0,0) to check whether inequation is satisfied or not. If it satisfies the inequation origin side is the required region else the other side is the solution.

Similarly, we repeat the steps for other inequation also and find the common region.

2x + 4y ≤ 120 → 2x + 4y = 120 (corresponding equation)

Two coordinates required to plot the equation are obtained as:

Put, x = 0 ⇒ y = 30 (0,30) - - - - first coordinate.

Put, y = 0 ⇒ x = 60 (60,0) - - - - second coordinate

x = 0 is the y-axis and y = 0 is the x-axis

Hence, we obtain a plot as shown in figure:

The shaded region in the above figure represents the region of a feasible solution.

Now to maximize our objective function, we need to find the coordinates of the corner points of the shaded region.

We can determine the coordinates graphically our by solving equations. But choose only those equations to solve which gives one of the corner coordinates of the feasible region.

Solving 2x + 4y = 120 and 2x + 2y = 80 gives (20, 20)

There are no other corners in the region obtained.

So maxima will occur at (20,20)

Now we have coordinates of the corner points so we will put them one by one to our objective function and will find at which point it is maximum.

∵ Z = 3x + 4y

∴ Z at (20,20) = 3×(20) + 4×(20) = 140

Note: As the region is unbounded, so we need to check whether maxima occurs or not.

For this we define inequation using the optimal function if the solution region of the inequation does not coincide with the feasible region, it means it has a maximum.

∴ inequation is : 3x + 4y > 140

Clearly, from the graph we observe that 3x + 4y>140 does not overlap with the feasible region

∴ Z is maximum at (20,20) and max. value is 140

∴ Z is maximum at x = 20 and y = 20 ; and max value is 140