Mathematical problems in mathematics A water conservancy survey team walked 5.3km upstream for the first time, 5.3km upstream the next day, 15km downstream the third day, and 5km downstream the fourth day. Where is the starting point of the survey team? How many kilometers away from the starting point?)

-5.3 * 2 + 15 + 5 = 9.4, 9.4km downstream
9.4km from the starting point

For the rational number XY, a new operation x * y = ax + by + C is defined, where a, B and C are constants, and on the right side of the equation are the usual addition and multiplication operations
1 * 2 = 9, (- 3) * 3 = 6, 0 * 1 = 2, find the value of (- 2) * 5, speed

∵x*y=ax+by+c
∴1*2=a+2b+c=9(x=1,y=2)
（-3）*3=-3a+3b+c=6(x=-3,y=3)
0*1=b+c=2(x=0,y=1)
∴a=2,b=5,c=-3
∴-2*5=-2*2+5*5-3=18
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Excuse me, how much is 23:10 minus 28 and 19:1 minus 1:20,

x. If y is a positive integer and 3 / x + 1 / y = 1, what is the minimum value of XY

3/ x+1/y=1
So 3 / x = 1-1 / y
Because y > = 2 (otherwise the original formula cannot have a solution in the range of positive integers)
So 1 / 2

How much does it cost to buy 20 bottles now? How much cheaper than the original price?

20 / (4 + 1) * 4 * 25 = 400 yuan
20 * 25-400 = 100 yuan
A: it costs 400 yuan to buy 20 bottles, 100 yuan cheaper

If x, Y > 0 and xy = 8, then the minimum value of 2 / x + 1 / y is 0

8(2/x+1/y)
=xy(2/x+1/y)
=2y+x>=2√(2y*x)=2√(2*8)=8
SO 2 / x + 1 / Y > = 1
Minimum = 1

A total of 52 people in our class, buy 4 boxes, just one box for each. How many boxes of drinks are there in each box?

52÷4
=13 (box)

One by two by one in three plus two by three by one in four Plus 9 times 10 times 11!
1 1 1 1
———— + ———— + …… + ———— + ————
1×2×3 2×3×4 8×9×10 9×10×11
PS：
Must be easy to calculate, 11 holiday I wait for the answer!

1/1*2*3+1/2*3*4+.+1/9*10*11
=(1/1)*[1/（2*3）]+(1/2)*[1/(3*4)]+.+(1/9)[1/(10*11)]
=(1/1)*(1/2-1/3)+(1/2)*(1/3-1/4)+.+(1/9)(1/10-1/11)
=1/(1*2)-1/(1*3)+1/(2*3)-1/(2*4)+.+1/(9*10)-1/(9*11)
=[1/(1*2)+1/(2*3)+.+1/(9*10)]-[1/(1*3）+1/（2*4）+.+1/（9*11）]
=（1-1/2+1/2-1/3+.1/9-1/10）-（1/2）*[2/(1*3）+2/（2*4）+.+2/（9*11）]
=1-1/10-(1/2)*[1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+.+1/8-1/10+1/9-1/11]
=1-1/10-(1/2)*（1+1/2-1/10-1/11）
=1-1/10-1/2-1/4+1/20+1/22
=27/110

On the map with a scale of 1:6000000, the distance between a and B is 25cm. A plane flies from a to B at 9:30 a.m. and arrives at 10:45 a.m?

10: 45-9:30 = 1 hour, 15 minutes = 1.25 hours, 25x60000 = 1500000000 cm = 1500 km, 1500 / 1.25 = 1200 km
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2005 + 2004-2003-2002 + 2001. + 8-7-6 + 5 + 4-3-2 + 1?

It's 2005
Except for the regular numbers after 2005, every four numbers add up to 0
2004 is an integral multiple of 4, so it's followed by 0

For the rational number XY, a new operation x * y = ax + by + C is defined, where a, B and C are constants, and on the right side of the equation are the usual addition and multiplication operations 1 * 2 = 9, (- 3) * 3 = 6, 0 * 1 = 2, find the value of (- 2) * 5, speed