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OA is A
As per the constraints given in the question. Robin earned more than twice this week than that of last week.
So if he earned X last week so this week earning is 2X+K
Now assuming he sold 12 laptops already. And is selling after 12 laptops in later weeks. For which he is getting 18$ each. His 20$ income is fixed for both.
So if he sold 15 laptops in last week.
He must have sell more than 30 laptops this week to earn such profit. And statement one is proving that.
So OA is A

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Given:
1. Working in Laptop company, Robin earns a fixed salary of $20 per week plus $6 per Laptop for the first 6 laptops he sells, $12 per Laptop for the next 6 Laptops he sells, and $18 per Laptop for every Laptop sold after first 12.
2. This week, he earned more than twice as much as he did last week.

Asked: If he sold a Laptops last week and b Laptops this week, which of the following statements must be true?

Robins's earning last week = $20 + $6a if a<=6; $56 + $12(a-6) if 6<a<=12; $128 +$18 (a-12) if 12<a
Robins's earning this week = $20 + $6b if b<=6; $56 + $12(b-6) if 6<b<=12; $128 +$18 (b-12) if 12<b

I. b>2a
Case I:
If a = 6
Robin's earning last week = $20 + $6*6 = $56
If b = 11
Robin's earning this week = $20 + $6*6 + $12 *5 = $116 > 2*$56 = $112
But b<2a
Case 2:
If a = 6
Robin's earning last week = $20 + $6*6 = $56
If b = 13
Robin's earning this week = $20 + $6*6 + $12 *6 + $18 = $146 > 2*$56 = $112
But b>2a
b> 2a is NOT NECESSARILY TRUE

II. b>a
This week, he earned more than twice as much as he did last week.
II. b>a MUST BE TRUE

III. b>3
For earning this week > 2*20 = 40
$20 + 6b > $40
6b > $20
b > 20/6 = 3 1/3 > 3
III. b>3 MUST BE TRUE

A. I only
B. II only
C. I and II
D. II and III
E. I, II, III

IMO D
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Answer:A

consider 3 cases:
CaseI:in which ,less than equal to 6 laptop
Earned this week >2 X earned last week
20+6b>2X(6a+20)
b>2a+3
Case II:Laptop selling between 6-12
20+36+b-6X12>2x(20+36+(a-6)X12)
b>2a-1.3(apprx)
Case III:Laptop selling >12
20+6*6+6*12+(b-12)*18>2*(20+6*6+6*12+(a-12)*18)
b>2a-apprx4.8
Combining all 3 cases ,b>2a
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Quote:

Working in Laptop company, Robin earns a fixed salary of $20 per week plus $6 per Laptop for the first 6 laptops he sells, $12 per Laptop for the next 6 Laptops he sells, and $18 per Laptop for every Laptop sold after first 12. This week, he earned more than twice as much as he did last week. If he sold a Laptops last week and b Laptops this week, which of the following statements must be true?

I. b>2a
II. b>a
III. b>3

A. I only
B. II only
C. I and II
D. II and III
E. I, II, III

Understand the difference here

Last week = 20+6+6+... so on.

This week=20+6+6+6+6+6+6+12+12.. so on.

The only way this week can be more than double the last week is if we have every 12 for every 6 in last week and at least one 18.. which is exactly as statement 1 b>2a.. Hence that is the only one true.

Statement 2 is clearly not must be true because we have 1 as must true.

Statement 3 is true but not always. It is true when a=0 but not otherwise. For example b=4 gives us 20+24=44 and if a=0 then it works but if a=1 the we have 26 and 44 is not more than double of 26.. Hence not always true.

Answer A
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