Linear Momentum and Collisions

Homework Statement

A bullet with a mass of 4.0 g and a speed of 661 m/s is fired at a block of wood with a mass of 0.075 kg. The block rests on a frictionless surface, and is thin enough that the bullet passes completely through it. Immediately after the bullet exits the block, the speed of the block is 25 m/s. What is the speed of the bullet when it exits the block?

m1 = 4g -> 0.004 kg
v1b = 661 m/s
v1a = ?
m2 = 0.075 kg
v2b = 0 m/s
v2a = 25 m/s

Homework Equations

m1v1b + m2v2b = m1v1a + m2v2a

The Attempt at a Solution

m1v1b + m2v2b = m1v1a + m2v2a
(.004)(661) = (.004)(v1a) + (.075)(25)
2.664 = (.004)(v1a) + 1.875
.789 = .004v1a
197.25 m/s = v1a

I've double checked this and gotten the same answer twice, but we have to use WebAssign, and the error it gives is "Your response is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error." which I don't understand? I don't see what's been done wrong, and there's other parts of the problem I can't continue to until this is done. Any help would greatly be appreciated!