but if we solve it like this:

subtract

x+y < 20

y < 20

x+y-y<0

x<0

then x will be certainly less than 20...

Why is this approach flawed ?

Bunuel wrote:

enigma123 wrote:

Is x < 20?

1. Sum of x and y is less than 20

2. y is less than 20

How come the answer is E and not A?

Consider two cases:

x=30 and y=-15;

x=15 and y=-15;

Both examples satisfy the statements and give different answers to the question whether x<20. Not sufficient.

Answer: E.