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Eight mathematical equations
1 Let the ship's own speed be x kilometers per hour (x+2)*3=(x-2)*3.5, and the distance between the two places be (x+2)*3=84.

Suppose the upstream time of the ship is x, then the downstream return time is 7.5-x.

(15-3) * x = (15+3) * (7.5-x) If x=4.5, the distance between the two places is (15-3)*4.5=54.

3 let the water velocity be X km/h and the still water velocity be 9 x km/h.

7.75 * (9x+x) = 9 * (9x-x)+1,and the distance between the two places is 7.75*(9x+x)= 155.

4 Suppose the annual interest rate is x5000 * (1+x) * (1+x) = 5500, and the annual interest rate is about 4.88%.

5 let the selling unit price be x 1500+0.25x=x to get x=2000.

6 Let the current speed be X km/h and the swimmer's own speed be A km/h (A does not need to be solved).

1/3+[(a-x)1/3+2]/(a+x) = 2/x gives x=3.

7 let the repayment of each installment be x yuan (10225-x) * (1+4.5%) = x=5225.

8 let the downhill time be x hours and the uphill time be y hours.

12x+(55/60-x) * 9 = 8 * (1.5-y)+4y12x = 4y gives x=0.25 y=0.75.

The distance from the school to the county seat is 8*( 1.5-y)+4y=9.