If x = 5 and c = 0.25, how do I find b in x/(x+b) = c?
Out of different methods, you may want to experiment with this:
x/(x + b) = c/1
Invert each side:
(x + b)/x = 1/c
x/x + b/x = 1/c
1 + b/x = 1/c
b/x = 1/c - 1
b = x(1/c - 1)
Substitute the values for x and c and simplify the
expression to an integer, or fraction, as appropriate.
In addition to greg1313 solution, you can also do this way
x/(x+b) = c
multiply both sides by (x + b)
(x + b)x/(x + b) = c(x + b)
(x + b) on the left side will be cancelled
x = c(x + b)
x = cx + cb
cb = x - cx
b = (x - cx)/c = (5 - (0.25)(5))/0.25 = 15