standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Task
One online source suggests that exploiting solar energy makes sense in an area that receives $9\frac{kWh}{m^2}$ of solar energy per day and does not make sense in an area that receives only $2\frac{kWh}{m^2}$ of solar energy per day. Does it make sense to exploit solar energy in Santa Rosa?


aligns: no