She would have a total of 40 stamps.
800 + 16x = 720 + 21x. Cancel out the 720 to get 80 + 16x = 21x. Subtract 16x from both sides to get 80 = 5x. Divide by 5 to get 16 = x, or x = 16. 16 hours. The charge is 800 + 16(16), = 720 + 21(16) which = 800 + 256 = 720 + 336 which = 1,056 = 1,056. Hope this helped!
Answer:
As per the statement;
The moon takes 28 days to orbit around the earth going at a distance of 2,413,000 km
From the above information,
The ratio between Distance and Days is; 
then,
the distance it takes for the moon to travel one day of its orbit around the earth we have;
km.
Therefore, the distance in kilometer does the moon cover in 1 day is 86178.57 km