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People's education printing plate seventh grade mathematics chapter 8 always reviews the answers.
Let the uphill from A to B be Xkm, the flat road be Ykm, and the downhill road be Zkm, then

X+Y+Z=3.3

X/3

+

Y/4

+

Z/5

=

5 1/60

Z/3

+

Y/4

+

X/5

=

53.4/60

20x+15y+12z = 51is obtained from Equation 2.

20Z+ 15Y+ 12X=53.4 is obtained from formula ③.

Z-X=0.3 from ⑤-④, then Z=X+0.3.

If Formula 6 is brought into Formula 1, X+Y+X+0.3=3.3, then Y=3-2X.

Bring the formula ⑥ ⑥ into the formula ⑥ and get 20x+15 (3-2x)+12 (x+0.3) = 51,so X= 1.2.

Y=0.6,Z= 1.5

So from A to B, it is 1.2km uphill, 0.6km flat slope and 1.5km downhill.