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