Here is a velocity vs time graph ... method. Let's just break the curve into many small rectangles where we assume the velocity is constant. Here I am calling the width of this rectangle dt ...

Velocity-time graphs ... under the graph. In the first 10 seconds this is the area of the triangle, ½ × 10 s × 40 m/s = 200 m. From 10 to 20 seconds this is the area of the rectangle, 10 ...