1.) x is 15.
Multiply the values on the same line. Equate and simplify.
6*x = 10*9
6x = 90
x = 15
2.) x is 16.
Do the same thing as the first number.
5*x = 8*(x-6)
5x = 8x - 48
3x = 48
x = 16
3.) The radius is 7.25.
The value 5 is the same on the opposite side.
5*5 = 2*x
25 = 2x
x = 12.5
Add x and 2 to get the circumference, then divide the result by 2 to get the radius.
r = (12.5 + 2) / 2 = 14.5 / 2 = 7.25
4.) x is 37.6.
Add the values in the same line, then multiply the result by the value outside the circle. Equate, transpose, and multiply.
14(20+14) = 10(10+x)
476 = 100 + 10x
376 = 10x
x = 37.6