Admission to a professional course is subject to the following condition: a) Marks in mathematics > = 60 b) Marks in Physics > = 50 c) Marks in Chemistry > = 40 d) Total in all three subjects > = 200

So:

X >= 60
Y >= 50
Z >= 40

Take 51 sheets of "graph-paper" (1+200 minus 60 minus 50 minus 40).

On the first sheet of paper, label the graph-title as 'Z=40'.
Label the X-axis from 60 to 200.
Label the Y-axis from 50 to 200.
Draw a straight line from (X=60 and Y=(200-60-40))
to (Y=50 and X=(200-50-40)).

On the second sheet of paper, label the graph-title as 'Z=41'.
Label the X-axis from 60 to 200.
Label the Y-axis from 50 to 200.
Draw a straight line from (X=60 and Y=(200-60-41))
to (Y=50 and X=(200-50-41)).

On the next sheet of paper, label the graph-title as 'Z=42'.
Label the X-axis from 60 to 200.
Label the Y-axis from 50 to 200.
Draw a straight line from (X=60 and Y=(200-60-42))
to (Y=50 and X=(200-50-42)).

On the second sheet of paper, label the graph-title as 'Z=43'.
Label the X-axis from 60 to 200.
Label the Y-axis from 50 to 200.
Draw a straight line from (X=60 and Y=(200-60-43))
to (Y=50 and X=(200-50-43).

Keep going for all 51 sheets of graph-paper.

Stack the 51 sheets on top of each other,
to visualize the "3-D" solution to the problem.

P.S. Don't tell your school-teacher that I helped you do your "homework".