Given:
1. Mrs. Stark paid $6,000 for 100 shares of stock A and $4,000 for 100 shares of stock B.
2. She later sold the same shares of both stocks, gaining 3/8 the amount she paid for stock B.

Asked: If the prices she had paid for the stocks had been reversed and everything else remained the same, then the net result would have been:

Stock A:
Number of shares = 100
Total amount invested = $6000
Purchase price = $6000/100 = $600

Stock B:
Number of shares = 100
Total amount invested = $4000
Purchase price = $4000/100 = $400

Total gain by selling all shares = 3/8 * $4000 = $1500

If price for shares had been reversed

Stock A:
Number of shares = 100
Total amount invested = $4000
Purchase price = $4000/100 = $400

Stock B:
Number of shares = 100
Total amount invested = $6000
Purchase price = $6000/100 = $600

Total gain by selling all shares = 3/8*$6000 = $2250

Net gain = $2250/$1500 = 3/2 = 1.5

IMO E
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Quick solution (time taken :40 secs)

net gain in the 1st transaction = 3/8 of 4000 = 1500 Rs.
now the prices are reversed. Since the number of shares is the same for Stock A and B, shares in Stock B were purchased at 6000 and stock A at 4000.
Net gain now = 3/8 * 6000 = 2250
the CP is same in both the cases. Since the profit increased, only one option depicts that IMO E

Net price per share of A = 6000/100 ; 60 and B = 4000/100 = 40
net gain of B ; 40*3/8 ; 15$
and when price are reversed net gain of B now becomes; 60*3/8 ; 22.5
so net result of gain ; 22.5/15 ; 1.5 $
IMO E