Q2

[kdimenno]

but why can't figs be the only kind of fruit the stand carries? i understand why the answer is tangerines, but it seems like figs would fit by the same logic.

[ManhattanPrepLSAT1]

This is a really important step in developing a feel for binary grouping.

Let's look at the difference between the two following constraints.

1. If the stand carries kiwis, then it does not carry pears.

2. If the stand does not carry tangerines, then it carries kiwis.


Let's look at the first one...

Put into formal notation

K --> ~P

This implies if the kiwis are in, then the pears are out. By contrapositive, if the pears are in, then the kiwis are out. It's possible according to this constraint though that both kiwis and pears are out. It's not possible, however, that both kiwis and pears are in.

Let's compare now to the second constraint...

Put into formal notation

~T --> K

This implies that if the tangerines are out, then the kiwis are in. By contrapositive, if the kiwis are out, then the tangerines are in. It's possible that both the kiwis and tangerines are in. It's not possible, however, that both the kiwis and tangerines are out.

I think of positive to negative
A --> ~B
as at most one, definitely not both.

I think of negative to positive
~A --> B
as at least one, and possibly both.

The reason why figs cannot be the only fruit the stand carries is because at least one of the tangerines and kiwis must also be carried. So, the answer must be either tangerines or kiwis and latter is not an available answer choice.