能被56整除,即能被7、8整除
要被8整除,末三位就要被8整除
98X被8整除,显然X=4
X19984要能被7整除,根据末位截三法
984 - X19
= 984-19 - 100X
= 965 - 100X
= 138*7 - 14*7X - 1 - 2X
= 7*(138+14X) - (2X+1)
即2X+1能被7整除,X显然 = 3
综上,319984能被56整除