Solution:

Assuming that during this period, the total profit is Y yuan, then:

When each piece is sold at X yuan, the profit of a single commodity is:

(x-30) yuan, * * * sold (100-x) pieces, so:

y=(x-30)( 100-x)

=-x？ + 130x-3000

=-(x-65)？ +7225

Obviously, when x=65, Y has a maximum value of 7225, so when the price is 65 yuan, its profit is the largest.

3.

Solution:

s=60t- 1.5t？

=- 1.5(t-20)？ +600

As can be seen from the above formula, when the time is less than 20 seconds, the plane is still taxiing, and when it is more than 20 seconds, the distance will be less than 600 seconds, but in actual operation, it is impossible for the plane to taxi 600 meters before returning. Therefore, when the taxiing time is 20 seconds and the distance is 600 meters, the plane can only taxi for 600 meters before stopping.