Three times of a number is 12.8 less than five times of it. Find the number (by solving the equation)

Three times of a number is 12.8 less than five times of it. Find the number (by solving the equation)

X*5-X*3=12.8
The solution will come out

Four times of a number plus three and two fifths equals 13.4. What's the number

Let x 3 and 2 / 5 = 17 / 5 13.4 = 67 / 5
4x+17/5=67/5
The solution is x = 5 / 2, which is 5 / 2

Three fifths of a number is two fourths more than 20% of the number. Find the number. (use the solution of the equation)
The wrong number is one fourth more!

Let this number be X
3/5x-20%x=1/4
0.6x-0.2x=0.25
0.4x=0.25
x=0.625
A: this number is 0.625, or 5 / 8
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One third of a number is 20 more than one fifth of 20. What is the number? Use the equation

Let this number be X
X/3-20×1/5=20
X/3-4=20
X/3=24
X=72
A: the number is 72

It is known that the left focus of ellipse C: x2a2 + y2b2 = 1 (a ＞ B ＞ 0) is f, C intersects with the straight line passing through the origin at two points a and B, connecting AF and BF. If | ab | = 10, | BF | = 8, cos ∠ Abf = 45, then the eccentricity of C is ()
A. 35B. 57C. 45D. 67

In the △ AFB, as shown in the figure, in the △ AFB, in the △ AFB, the | ab | ab | = 10, | BF