Based on the classic rule of thumb, what would be the approximate weight gain of a person who consumes an excess of 500 kcalories daily for one month

To gain 1 pound every day, you need around 21000 excess calories. So if you consume an excess of 500k calories daily, we can calculate the weight gain by:

weight gain = (30 days) (500,000 calories/day) : (21,000 calories/1lb)

weight gain = 71 lbs

In consuming an excess of 500k calories daily, you will gain 71 lbs in a month.