The shape of the room is not a square. Pythagorean Theorem: 17^2+17^2= 578, square root 578 and you'll get 24.04. The length of the diagonal of the floor of the room in the plan should have been 24.04 in order to be a square.
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.