The specific answer is as follows:

The unit price of toffee is equal to:

(20.4× 4-17× 3) ÷1= (81.6-51) ÷1= 30.6 yuan.

The price of 6.8kg toffee is equal to:

30.6× 6.8 = 208.08 (yuan)

So the unit price of toffee is 30.6 yuan, and 6.8 kg of toffee is 208.08 yuan.

Mixed computing:

If the first-level operation and the second-level operation exist at the same time, the second-level operation is calculated first. If there are one, two and three operations at the same time (i.e. power, root and logarithm operations), the third operation is calculated first, and then the other two levels are calculated.

If there are brackets, count the numbers in brackets first (no matter what level, count first) The third level should be counted in brackets, and then the second and first levels should be counted.