Try to contact examples in life, make up two practical problems, and list the corresponding binary linear equations

Try to contact examples in life, make up two practical problems, and list the corresponding binary linear equations

5.12 during the earthquake relief work in Sichuan Province, we went to two PLA rescue units in the rescue activities of a certain place. We have known that 210 people have been sent to the two units, and one of them has 10 more people than the other. Q: how many people are there in each of the two PLA rescue units?
There are x men in one unit and Y men in the other
X + y = 210
X-Y=10
X = 110. Y = 100
The two PLA units have 110 and 100 members respectively

x²-17x+51=0

x²-17x+51=0
a=1,b=-17,c=51
b²-4ac=289-4×1×51=85
x=(17±√85)/2
x1=(17+√85)/2,x2=(17-√85)/2

30% of a number plus 21 equals itself. What's the number?

21 △ 1-30%, = 21 △ 0.7, = 30; a: this number is 30