Non Standard Word problems

[H287]

How does one approach a word problem like the below? Its from MGMAT CAT.
Pls. suggest..

Thanks!

As a bicycle salesperson, Norman earns a fixed salary of $20 per week plus $6 per bicycle for the first six bicycles he sells, $12 per bicycle for the next six bicycles he sells, and $18 per bicycle for every bicycle sold after the first 12. This week, Norman earned more than twice as much as he did last week. If he sold x bicycles last week and y bicycles this week, which of the following statements must be true?
I. y > 2x
II. y > x
III. y > 3
I only
II only
I and II
II and III
I, II, and III

Ans: D

[venubab]

1. Take X = 6 Amount = 56

Take Y = 10 Amount = 104>102

Y<2x

3. X=0 Amount=20
Y=3 Amount=38<40
Y=4 Amount=44>40
Y>3

2 and 3

[life is a test]

II only

1. Take X = 6 Amount = 36

Take Y = 10 Amount = 84>72

Y<2x

3. X=1 Amount=6
Y=3 Amount=18>12
Y=3

total earned = 20 + (6 bikes @6) + (6 bikes @12) + 18 (x-12)

20 is the same regardless of how many bikes he sells. The rate of commission doubles for bikes over 6 (for 6 bikes) so if his earnings doubled its because he sold >3 bikes at the higher rate of commission. To get to the higher rate of mission he must have sold 6 bikes in the first week and >9 bikes in the second week.

hope that helps.