Is the number of seconds required to travel d1 feet at r1 feet per second greater than the number of seconds required to travel d2 feet at r2 feet per second?
(1) d1 is 30 greater than d2.
(2) r1 is 30 greater than r2.
In the original condition, there are 4 variables(r1,d1,r2,d2), which should match with the number of equations. So you need 4 more equations. For 1) 1 equation, for 2) 1 equation, which is likely to make E the answer. In 1) & 2), d1/r1>d2/r2? becomes (30+d2)/(30+r2)>d2/r2?, which also develops into (30+d2)r2>(30+r2)d2?. From 30r2+d2r2>30d2+r2d2?, delete r2d2 in the both equations and they become 30r2>30d2?. So, you cannot get the answer from r2>d2?, which is not sufficient. Therefore, the answer is E.
-> For cases where we need 3 more equations, such as original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 80% chance that E is the answer (especially about 90% of 2 by 2 questions where there are more than 3 variables), while C has 15% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since E is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, C or D.
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