Answer :
7840-group all the same denominations together like this: 6 thousand + 1 thousand is 7 thousand, 5 hundred +3 hundred is 8 hundred and 4 tens 4 times 10, means count by tens.
7840-group all the same denominations together like this: 6 thousand + 1 thousand is 7 thousand, 5 hundred +3 hundred is 8 hundred and 4 tens 4 times 10, means count by tens.