The 300@$95 cost ..3167 each

The 100@$45 cost .45 each

If you buy 2*300@2*$95 it will cost $190

If you buy 2*100@2*$45 it will cost $90

You will have 800@ $280,= The lowest cost.

You are a biological technician in a lab and you need to order 800 petri dishes.

A pack of 100 petri dishes costs $45.

A pack of 300 petri dishes costs $95.

The lowest cost you could spend on the order:

2 packs of 300 dishes + 2 packs of 100 dishes cost $190 + $90 = $280

Answers provided:

A. $280  B. $285  C. $320  D. $360  E. $450

Answer choice: A. $280

you can only order Petri dishes in packs of either 100 pks or 300 pks

.

Possible combinations of packs include the following:

Each time you buy a 300 pack of petri dishes it only costs $95. 

Compare that with buying 3 lots of 100 petri dishes at $45 each making $135. 

Buying 300 packs is at a much better rate, but you cannot buy all of the  

800 that way.  

It would have been fine if you needed 900 because 900 = 3 * 300. 

But all you can do at the good rate is 2*300 which will cost $190 ...(1) 

You need 800, not 600 so you have to buy the other 200 at the bad rate. 

That will cost 2 * 45 = $90  .................(2) 

The least total cost is 190 + 90 = $280 which is the answer asked for. 

Note: The cost of 3 * 300 would have been $285  

So in reality your answer would be reasonable, but was not the lowest cost.

You might have been happy to accept the extra 100 petri dishes for only $5 extra 

You are assuming getting 3 300 packs is cheaper. If you'll use the extra hundred, it might be the best buy. But if you only need 800, what happens if you get 2 300 packs and 2 100 packs?

You need 800

Order 600 for $190 (95x2)

Order 200 for $90 (45x2)

I have NO IDEA what you are doing wrong as you have no work shown. I can assure you that the answer is indeed $280