Each statement on it's own is insufficient, as it does not provide sufficient info about all 4 variables.
Both Statements together:
a < c
b < d (You can flip an inequality like this)
When combining 2 inequalities make sure that:
1. You have oriented them such that they have the same symbol ( < , >, etc)
2. You are adding them. DO NOT subtract.
Adding these two inequalities gives us :
a + b < c + d
This is a third inequality. But to get it to answer our prompt, we need to manipulate it a bit more. Technically you can keep adding inequalities to eachother as long as you follow those 2 rules.
Thus we could add our inequality b < d to the above again, to get to our prompt.
a + 2b < c + 2d
For a quick revision of the basic rules of dealing with inequalities, refer to this link:
https://www.platinumgmat.com/gmat_study_ ... equalities~Abhay
Believe those who are seeking the truth. Doubt those who find it. -- Andre Gide
