I've tried several equations to no avail. Please help
"A person changed $200 into five- and ten- dollar bills. The person then had 28 bills in all. How many ten-dollar bills were there? What was the value of the five-dollar bills?"
Does this make any sense? My teacher showed me this set-up which was totally different from yours.
Let 5f = number of five dollar bills
Let 10(28-f) = number of ten-dollar bills
200 = 5f + 10(28-f)
200 = 5f + 280-10f
200 - 280 = 5f - 10f
-80 = -5f
f = 16 number of five-dollar bills
28 - 16 = 12 number of ten-dollar bills
$120.00 value of ten-dollar bills
$80.00 value of five-dollar bills
"Let 5f = number of five dollar bills": that' s not the number of $5 bills, it's the VALUE of the $5 bills.
Did your teacher not show this way:
Let f = number of $5 bills; then 28-f = number of $10 bills?
Then proceeded to:
5f + 10(28 - f) = 200