4 x 10exp4 x 3ms-1 = 120000 Kg ms-1

That's the momentum of the moving truck before collision.

2 x 10exp4 x 0ms-1 = 0 Kg ms-1 ( by stationary I take it, it means 0 ?)

That's the momentum of the stationary truck before collision.

Yes, it is zero.

Total momentum = 120000 + 0 = 120000

Before collision.

Total momentum after collision is equal to total momentum before collision if you exclude all other factors?

Yes. Momentum is always equal before and asfter collisions.

Speed ?

The total mass now of the combined two trucks is

(4*10^4 +2*10^4) = 6*10^4 kg

So, mV = (6*10^4)V

And that is equal to 120000 or 12*10^4.

Hence,

(6*10^4)V = 12*10^4

V = (12*10^4)/(6*10^4) = 2 m/sec.

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Here, I am not very sure because I am rusty in Physics.

KE = (1/2)mV^2

Before collision,

KE = (1/2)(4*10^4)(3^2) +0 = 18*10^4 joules

After collision,

KE = (1/2)(6*10^4)(2^2) = 12*10^4 joules.