Mrs.Kennedy can divide 315 by 10 and then when she gets her answer she can round up to get her answer.
If order doesn't matter (e.g. all the positions in the committee are essentially identical) then the number of possible choices is

.
%error=100(estimate-actual)/actual
%e=100(50-40)/40=25%
can't see the whole question but...
40(60/100)=24
so 24/40=3/5 carry a phone.
24 students carry a phone.
Answer:
Unreduced: (4) + (5 • x) + (-9) + (4 • x)
Reduced: 9x – 5