Lea buys 20 picture frames. some are 4 in. by 6 in. frames which cost $5.80 each, and the rest are 5 in. by 7 in. frames that cost $9.60 each. she spends a total of $80.80.
which equation models this situation?
let f represent the number of 4 in. by 6 in. frames she buys.
a.
9.6f + 5.8(20) = 80.8
b.
5.8f + 9.6(20 – f) = 80.8
c.
(5.8 + 9.6)(20 – f) = 80.8
d.
9.6f + 5.8(f + 20) = 80.8

To find the answer we have to slowly analyze the command.
 For surely on the one side we want to have total cash spent, so 80.8$
We know that there are 2 types of frames 5,80$ and 9.60$.
We know too, that f is the number of frames 5.80$ bought.
Simply, to find the cash spent on those frames we have to just mulitply 5.80$ times f, so we obtain .
We can say that number of bigger frames is 20-f.
So to find cash spent on bigger frames we have to multiply 9.60$ times (20-f), so

Now we can just add cost of all frames together

And we know that is equal to 80.8, so
=80.8

As we can see, the answer is b.

3.(a + 2b)
= (3a + 6b)