100+80%(x- 100) When the total price to be paid by Party A exceeds 50+90% (X-50), it is more economical for Party B to go to the store.

100+0.8x-80>50+0.9x-45

-0. 1x>- 15

x< 150

Answer: Go to Store B when the purchase price is lower than 150 yuan, and go to Store A when it is higher than 150 yuan.

2. Solution: Set up a school to buy X computers.

( 1)6000+( 1-25%×6000)(X- 1)A 5。

Similarly, (2) 6000+(1-25% × 6000) (x-1) > (1-20%) 6000 x.

Solve the inequality and get x < 5.

(3)6000+( 1-25%×6000)(X- 1)=( 1-20%)6000 X。

Solve the equation and get x = 5.

3. Solution: Suppose you want to buy X cups.

When A is economical, it is 4× 20+5 (x-4) A < 92% (4× 20+5x) B.

Solve inequality. Get x < 3.4.

According to the meaning of the question, the number of teacups should be an integer.

So x < 3.4 (x is an integer), that is, x≤3.

Answer: When the number of cups purchased is less than or equal to 3, Store A is economical; If it is greater than 3, store B is economical.