#### Answer

The best estimate is b. \$$310$.

#### Work Step by Step

We know that the most expensive calculator is approximately \$$40$, and the least expensive one is approximately \$$20$, and half ($5$) calculators cost approximately \$$30$.
The total cost of those calculators is:
$20+5*30+40=210$
The average cost of those $7$ calculators is $270\div7=30$
If that was also the average cost of the $10$ calculators, then the total price would be:
$10*30=300$
In the cheapest possible case, the remaining 3 calculators would all cost \$$20$, which would make the total price:
$210+3*20=270$
In the most expensive case, the remaining calculators would all cost \$$40$, which would make the total price:
$210+3*40=330$
The four possible answers for the best estimate of the price are:
a. \$$240$
b. \$$310$
c. \$$345$
d. \$$355$
Out of these four, looking at all the possible sums, the best estimate is b. \$$310$.