Chapter 6 - Section 6.7 - Quadratic Equations and Problem Solving - Exercise Set - Page 468: 42

16 phones

$C=n*(n-1)/2$ $C=120$ $120=n*(n-1)/2$ $120*2=n*(n-1)/2*2$ $240 = n*(n-1)$ $n^2-n-240=0$ $n^2-16n+15n-240=0$ $n(n-16)+15(n-16)=0$ $(n-16)(n+15)=0$ $n-16=0$ $n=16$ $n+15=0$ $n=-15$ (invalid since we can't have a negative number of phones)