From GMATPrep

Ann deposited money into two new accounts, A and B.
Account A earns 5 percent simple annual interest and
account B earns 8 percent simple annual interest. If
there were no other transactions in the two accounts,
then the amount of interest that account B earned in
the first year was how many dollars greater than the
amount of interest that account A earned in the first
year?

(1) Ann deposited $200 more in account B than in
account A.

(2) The total amount of interest that the two accounts
earned in the first year was $120.

This is what I have….
Rephrpase: What is IB – IA?

Since I = PNR

NA = NB = 1
RA = .05
RB = .08

IA = PA*NA*RA = PA*(.05)
IB = PA*NA*RA = PA*(.08)

IB-IA = PA*(.08) - PA*(.05)

1) PB = 200 + A ; Insufficient
2) IA + IB = 200; Insufficient
3) Plugging in 1) and 2) still leaves me with IB and PA, still gets insufficient for me.

Official Answer is C.

I realized what I did wrong. All you need to do to complete the problem is plug in values for IA and IB in IA + IB = 200 and solve for the unknown variables to eventually get you the result of IB-IA.

Stupid mistake:!: Can't make errors like this on the real deal....

just remember that you don't actually have to solve systems like this one, and, indeed, it's a complete waste of time to solve them. all you have to do is realize that you can solve them.

statement 1: Pb = Pa + 200
insufficient by itself

statement 2: 0.05Pa + 0.08Pb = 120
insufficient by itself

both statements together: sufficient
(both statements are linear equations, and neither one is a multiple of the other; therefore, the 2x2 system has a unique solution)
there's no need to actually solve the system; all you have to do is note that you can solve it.