Math problem I'll give you a lot of points. Come on, math genius China has a vast territory, 5200km from east to west. The distance between East and West is 52 / 55 of that between North and south? There is a set of gears that engage with each other (1) The pinion has 28 teeth, which is one fifth of the number of big gears. How many teeth does the big gear have? (2) The big gear turns 80 cycles per minute, which is 4 / 5 less than that of the small gear. How many cycles per minute does the small gear turn? Using equation Write the equivalent relation

Math problem I'll give you a lot of points. Come on, math genius China has a vast territory, 5200km from east to west. The distance between East and West is 52 / 55 of that between North and south? There is a set of gears that engage with each other (1) The pinion has 28 teeth, which is one fifth of the number of big gears. How many teeth does the big gear have? (2) The big gear turns 80 cycles per minute, which is 4 / 5 less than that of the small gear. How many cycles per minute does the small gear turn? Using equation Write the equivalent relation

China has a vast territory, 5200km from east to west. The distance between East and West is 52 / 55 of that between North and south?
The distance between North and south is x km
（52÷55）X=5200
The distance between East and West is (52 △ 55) km = 5500 km from north to south
(1) The pinion has 28 teeth, which is one fifth of the number of big gears. How many teeth does the big gear have?
Let the big gear have X teeth
（1÷5）X=28
Big gear (1 △ 5) = small gear 28
(2) The big gear turns 80 cycles per minute, which is 4 / 5 less than that of the small gear. How many cycles per minute does the small gear turn?
Set the pinion to rotate x revolutions per minute
[（1-4）×5]X=80
Pinion [(1-4) × 5] turns = big gear 80 turns

Let f (x) be a function defined on (0,1) and satisfy: ① for any x ∈ (0,1), f (x) > 0; ② for any x1, X2X ∈ (0,1), f (x1) / F (x2) + F (1-x1) / F (1-x2) ≤ 2, then for function f (x), f (x) has:
(1) for any x ∈ (0,1), f (x) > F (1-x); (2) for any x ∈ (0,1), f (x) = f (1-x); (3) for any x ∈ (0,1), f '(x) = 0; (4) when x ∈ (0,1), the function y = f (x) / x + X is a decreasing function____

Select 2 and 3 with exclusion method
We assume that f (x) is a constant function, where f (x1) / F (x2) + F (1-x1) / F (1-x2) = 2
The first and fourth items can be excluded
2. Three terms are equivalent, so choose two or three

It takes ten days for a and B to do something together, and 15 days for a to do it alone. How many days does it take for B to do it alone

Then it will take X days for B to do it alone
1/x+1/15=1/10
x=30
B. It takes 30 days to do it alone

Car a and car B leave from city a and city B at the same time, meet each other seven hours later, and then drive forward for two hours. At this time, car a is 240 kilometers away from city B, and car B is 360 kilometers away from city A. how many kilometers are there between city a and city B?

(240 + 360) △ 7-2) × 7 = 600 △ 5 × 7 = 120 × 7 = 840 (km). A: the distance between a and B is 840 km