Look at the pattern of shaded numbers. use place value to describe how the number change.
501 is shaded,502,503,504,505,506,507,508,509,510,511,512,513 is shaded,514,515,516,517,518,519,520,521,522,523,524,525 is shaded,526,527,528,529,530,531,532,533,534,535,536,537 is shaded,538,539,540,541,542,543,544,545, 546,547,548,549 is shaded,550