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I don't understand how the following problem can be solved. There's not enough info.

This is what the book says about it, but it fails to explain how disc D is 1.0 kg:

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    Draw the free body diagram on D. You should find that $T_3$ balances the gravitational force. – 2017-02-28

1 Answers 1

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The tensions are caused, as you go up the cord, by the incresing number of disks.

If the masses are $a,b,c,d$, then you have $$ (a+b+c+d)g=98N,\ (b+c+d)g=58.8N,\ (c+d)g=49N,\ dg=9.8N. $$ Now you can solve the last equation to find $d$, then the previous one to find $c$, and so on.