Question

On January 1t 2014, approximately 450 thousand buildings in the United Statcs (US) had sola panels. This number increased by a total of about 180 thousand over the next 12 months Assuming a constant rate of change, approximatcly how many months after January 1t 2014 would 900 thousand buildings in the US have solar panels?

Answer 1

Or, putting it a little differently - each month the number increases by 180,000/12, or 15,000 buildings/mo.

The increase between the two building populations is 900,000 - 450,000 = 450,000. So it would take an addtional 450,000/15,000 = 30 months to increase from 450,000 to 900,000.