Well it is a simple percentage question that we have to solve.
As we know that William made 620 by baby sitting and we also know that he made 340 per cent more next year so we can simply calculate 340 per cent of 620 and then add that percentage value to 620 to calculate the final answer.
So 340 / 100 * 620 equals to 2108 so the final answer is 2108 + 620 which becomes 2728.
As you increase the subintervals the area will be closer and closer to the real value. In other words your approximation gets better.
As you increase the intervals, there will be more rectanagles and the added area of these rectangles are converging towards the actual area under the curve.
