So the amount of money a store needs is: 477x

The amount of money needed by store B is: 530 * 3+(x-3) * 530 * 80% = 318+424x;

477x-(3 18+424x)>=0, then x & gt=6.

When the jewelry to be bought is more than 6 grams, it is more cost-effective to buy it in store B.

The second question:

Team a needs x days to complete, while team b needs (x+5) days to complete.

4 *( 1/x+ 1/(x+5))+(x-4)/(x+5)= 1

X = 20 days spent by team a.

X+5 = 25 days-days spent by team B.

Under the condition of not delaying the construction period, choose one or three schemes.

(1) A team completes the project alone, and it costs1.5 * 20 = 300,000 yuan.

(3) Party A and Party B cooperate for 4 days, and the rest is completed by Party B alone, (1.5+1.1) * 4+1.1* (20-4) = 280,000 yuan.

Option ③

Question 3: Open a shop and buy X TV sets and Y washing machines. Then there are countable equations.

x+y= 100。 = y/2 1800 * x+ 1500 * y & lt； = 16 1800

Can solve 100

So there are six options.

Profit = 200 * x+100 * y = 200 * x+100 * (100-x) =10000+100 * x.

The bigger the x, the greater the profit.

So buying 39 TV series, 6 1 washing machine has the biggest profit.