IMPOSSIBLE PHYSICS PROBLEM!

Two packages at UPS start sliding down the 20 degree ramp shown in the figure. Package A has a mass of 6.00kg and a coefficient of kinetic friction of 0.220. Package B has a mass of 8.00kg and a coefficient of kinetic friction of 0.170.

How long does it take package A to reach the bottom?

http://session.masteringphysics.com/problemAsset/1073551/3/knight_Figure_08_25.jpg

^ there is picture of the diagram

I keep getting 2.34 seconds, but it says I'm wrong. Can anyone tell me what they got and explain why?

thanks!

Before doing any math, let's look at the problem from this perspective: We need to find how fast it takes package A to reach the bottom. Package A will accelerate down the ramp along with Package B. BOTH Package A AND Package B MUST have the same acceleration down the ramp. Since Pacakge B is bigger and has less kinetric friction (0.170 compared to 0.220), it's going to push Package A down the ramp. Both packages are thus going down the ramp at the same speed. By knowing the acceleration of both packages, we can find how long it'll take Package A to reach the bottom (if we were told to find the time it took Package B to travel down, it would take longer than the given 2 m since Package B is behind Package A).
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PACKAGE A:

First, draw a free body diagram of Package A. If drawn correctly, our equations are :
ΣFx = max (the sum of the forces in the direction of motion equals the product of mass and acceleration)

(1) m1gsin(20°) + Fb - Ff_A = m1a
where:
m1 = mass of Package A (6 kg)
Fb = the force of Package B on Package A (remember, Package B is pushing Package A so a pushing force is present)
Ff_A = the frictional force on Package A
a = acceleration

ΣFy = 0
(2) N - m1gcos(20°) = 0

However, Ff_A = μk1*N = μk1*m1gcos(20°)