if 8 oranges cost 9 dollars how much does seven cost

Question

Eight oranges cost 9 dollars how much does 15 oranges cost

Answer 1

This is a problem about proportions. If 8 oranges cost $9, then one cost 8 times less: 9/8 dollars.
7 oranges cost 7 times as much: 7*9/8=63/8=$7.88 ($7 and 7/8 or $7.875 to the nearest cent); and 15 oranges cost 15*9/8=135/8=$16.88 ($16.875 rounded to nearest cent).
(If it had been 16 oranges it would have been twice as much as 8 oranges=$18. One orange costs $9/8=$1.125 ($1.13), and we can see that we can find the cost of 7 oranges by taking away the cost of one from the cost of 8: $9-$1.125=$7.875; and the cost of 15 by taking the cost of one from the cost of 16: $18-$1.125=$16.875.)