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A: 100+200+5(a-10)=5a+250
B: 150+150+10(b-10)=10b+200

Avg=0.5(5a+10b+450)
equals
20(a+b)

20(a+b)=5a+10b+450
=>15a+10b=450
=>3a+2b=90

Options-10: 5,10,12,15,17,20
a=20, b=15
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Cost X = 100 + 20x10 + 5 (X-10)

Cost Y = 150 + 10x10 + 10 (Y-10)


Average = (Cost X + Cost Y)/ (X+Y) = 20
450 + 5X + 10Y = 20X + 20Y
3X + 2Y = 90
X +2/3*Y = 30
=> Y must be divisible by 3 = 15, 27, 30

Y= 15; X = 20
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­There can be an alternate method which i used : 

For supplier X : 100 (fixed) + 20 (10) + x = 100 + 200 + y = 300 + x
For Y : 150 (fixed) + 15 (10) +  y = 150 + 150 + y = 300 + y

Now we know average unit price is 20 $  i.e,  300 + x + 300 + y / no. of items = 20
This also gives an estimation that total qty. combining x and y should be greater than 30.
I started to look for combination of 35 and just verified where i can get 20$ unit price.

Prepared additional table for additional prices (beyond 10 qty.) below 



300 + (50) + 300 + (50) / 15 + 20 = 700/35 = 20

P.S : This method can be inefficient if table have more fields but considering the current question, using above it can be solved in lesser time.
 ­
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­3x+2y=40 where x and y are the excess units of X and Y
x = (40-2y)/3

Is x is an integer, 40-2y should be a multiple of 3..

y=5, x=10 ; total 15, 20
y=8, x= 8 ; total 18 ,26
y=11, x=6.. you get the pattern.. y=5,8,11,14.. x=10,8,6,4,2...



 ­
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Dear experts gmatophobia Bunuel KarishmaB
It is given, the combined average is 20. when we talk about "combine avg" then it means we are taking the avg of a gp in which elements are avg itself. But here, we are taking avg of total cost for X and Y as 20, instead avg cost per item of X and avg cost per item of Y as 20 ??

kindly clear my doubt.
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Can,Will
Dear experts gmatophobia Bunuel KarishmaB
It is given, the combined average is 20. when we talk about "combine avg" then it means we are taking the avg of a gp in which elements are avg itself. But here, we are taking avg of total cost for X and Y as 20, instead avg cost per item of X and avg cost per item of Y as 20 ??

kindly clear my doubt.

We are given "... the combined average cost for the units was $20 per unit."
We are given that the average of the cost per item is $20. The individual costs are $5, $10 and $30 per item. We have taken them in quantities such that the average per item comes out to be $20. Does that clear your doubt?

I have discussed the weighted args approach using a scale for this question here: https://youtu.be/waWVH6zPj-4
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The highlighted should be 10x15
Garvesh
Atleast 10 units from each product means min. 10 products of X and 10 products of Y.
Cost = Fixed cost + Cost of first 10 products + Cost of remaining products.
For X,
Cost = 100 + 20x10 + 5 (X-10) ----[1]
For Y,
Cost = 150 + 10x10 + 10 (Y-10) ----[2]

Avg price per unit is 20.

Therefore,

([1] + [2])/ (X+Y) = 20

On solving above equation we get ,
3X + 2Y = 90

Hence from answer options we have X = 20 , Y = 15.­
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