HC VERMA Chapter-3 Volume-1Q.42I know the solution to the question. But the problem is that horizontal range for the projectile should be x=60cm and vertical height y=30cm. The answer will be different this way. When x=40cm and y=20cm then the answer comes. Why is horizontal range=40cm and vertical range=20cm?

Vertical range will be =30, but the horizontal range will be less than 60 cm. To calculate the minimum speed, you have to assume that it just touches the edge of the lowermost stair. So, x=40, y=-20 point will be a point on the projectile. You have to calculate the answer based on that. Any velocity less than that and the object will hit the lowermost stair.
For this case, the horizontal range will be less than 60, as the time taken to travel last 10 cm, will be less, as vertical velocity is the highest in the last phase. The vertical range will obviously be 30cm.