Which digit should replace A in the units place of the number 987,654,32A so that the number is divisible by 7?

Just try 0 divide by 7 if not try 1 divide by 7 if not try 2 divide by 7 if not......

(if it is 0 1 2 there will be a second one that will work)

let's say A=0

987,654,320/7 = 141093474.286

the .286 represents that we are 1 - 0.286 = 0.714 off from making 987,654,320 to be divisible by 7 wholly, therefore:

987,654,320 + 7 * 0.714 = 987,654,325