Let's just use some common sense reasoning, and we'll get close enough.  We'll 
keep the math operations simple at all times.  It's
the reasoning - the "numerical sense" - that takes practice.

(a) There are about 280 million men, women and children in the U. S.
    About half of them are adults. (that's 140 million.)
    About a fourth of all adults have cellular phones.  (35 million.)
    So there are about 35 million cellular phones in the U. S.

(b) A typical cellular phone breaks down in about two years.
    If there are 35 million cellular phones in the U. S., then every
       two years about 35 million phones will need to be replaced.
       So every year, about 17.5 million phones will be sold.
       there are 365 days in a year, so each day about 
       17,500,000/365 = 47,945 phones will be sold.

Now, since I've been making broad, common-sense estimates of numbers throughout 
this reasoning, it doesn't make sense to count the answers I get down to the 
single digits.  That's false precision.  So I estimate that about 50,000 phones 
will be sold.

(c) We've estimated there are about 35,000,000 cellular phones in the U. S.  I 
hear on television all the time about service deals that are around $50 per month 
- plus you get a free phone.  If every cellular phone were on that plan, then the 
cellular phone companies will make $50 per month per phone, or $50/mo. * 12 mo./yr 
= $600 per phone per year.  That would mean a gross income of 

   $600 * 35,000,000 = $21,000,000,000.  

The phone companies get no profit from selling the phones, because they're giving 
away the phones for free for those people who sign up to these plans.  So I 
estimate that the cellular phone companies will make about $20 billion in the 
United States this year.