1. If the cost price is X, the price is (1+0.6)x, and 30.6 yuan is 75% off the price, that is, 0.75( 1+0.6)x, so the equation is:

0.75( 1+0.6)x=30.6

(0.75+0.45)x=30.6

12x=30.6

x=25.5

Then the cost price is 25.5 yuan, and you can still earn 30.6-25.5=5. 1 yuan after the price reduction.

2. Assuming the pricing unit is 1, we can get:

A: After the first price reduction, it is 1-0.2, and after the second price reduction, it is (1-0.2) * (1-0.2) = 0.64.

B: After one-time price reduction, 1-0.4=0.6.

C: 1-0.3 after the first price reduction and (1-0.1) * (1-0.3) = 0.63 after the second price reduction.

So you can see that it is the most cost-effective to buy in supermarket B.

3. If the principal is X yuan, then his interest is 0.0225x and interest tax is 0.2(0.0225x), then the after-tax interest should be (1-0.2)0.0225x, and the principal is the principal amount of 8 144 yuan, so the equation is as follows:

( 1-0.2)0.0225 x+x = 8 144

0.0 18x+x=8 144

1.0 18x=8 144

x=8000

So his principal should be 8000 yuan.