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[SOLVED] Newtons Second Law - Simple Mechanics

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I've solved (i) to get the following values...

a= 2.5 ms^-2

T= 3.75 N

I want to inquire about part (ii)

What I want to know is that since acceleration of A was 2.5 then the acceleration of B should be -2.5 ... no?

I ask this because, for part (ii) I've got the correct answer of 'mass' by using

T-10m=2.5m

3.75/12.5=m

m=0.3 kg

But if I use -2.5 then the answer comes out to be m=0.5 kg.

What am I doing wrong?

Thanks!