Algebra

Let's set variables in order to solve this problems.

Let's let c be the constant added to each number. 

We get $60+c, 120+c, 160+c$

Since all the terms form an geometric series, we can write the formula

$\frac{60+c}{120+c} = \frac{120+c}{160+c}$

Now, we simplfy solve for c. 

$(c+160)(60+c)=(c+120)(120+c)$

Distributing everything, combining all like terms, and moving everything to one side, we get

$-20c-4800=0$

$c=-240$

We plug this back in, and we get the series $-180, -120, -80$

This has a common ratio of $180/120=3/2$

So 3/2 is our answer, 

Thanks! :)