Let the area of the wall be x.
It says Anil had painted 25% of the wall. So it becomes 25/100 of x or 0.25x.
After Umar painted 30 sq. m of the wall, in addition to the 0.25x of Anil painted, the wall is finished for 40%. (because it says 60% of the wall is left unpainted).
So the equation can be written as:
0.25x + 30 sq.m = 40/100 of x or 0.40x.
Solve the above equation for x.
i.e. 0.25x + 30 sq. m = 0.40x
0.40x-0.25x = 30 sq m
0.15x = 30 sq. m
x = 30 sq.m / 0.15
x = 200 sq.m.
It means the total wall area is 200 sq.m.
Since 25% of this area is painted by Anil, the area painted by Anil is 0.25*200 sq.m = 50 sq.m.