Solution: Let the unit cost be 1 and the ex-factory price of 1 month be a, then the ex-factory price of February is 0.9a According to the meaning of the question:

The unit profit in January is: a- 1.

The unit profit in February is 0.9a- 1.

Let 1 month sales be 10, and then February sales be 18, so the total profit increased in February compared with 1 month is:

18(0.9a- 1)- 10(a- 1)= 6.2a-8

And because the profit of each product sold in 1 month is 25% of the ex-factory price, so a- 1=0.25a, the solution is: a=4/3.

So the growth rate in February is: (6.2a-8)/10 (a-1) = (24.8/3)-8/(13) = (24.8-24)/1.