= Solution
The central estimate is $c=4.15/1.93-1=1.15026$. With independent input standard errors, the <ratio-minus-one uncertainty> from first-order <uncertainty propagation> is
$$
s_c^2=\left(\frac1b\right)^2s_a^2+\left(\frac a{b^2}\right)^2s_b^2,
\qquad\boxed{c=1.150\pm0.063\quad\text{(standard error)}.}
$$
The four independent reported results cluster near1.21, within about one quoted standard error of this estimate, so \b[they are consistent at this precision]. Their tight clustering is not evidence that this measurement has the same precision. If the measurements of a and b were correlated, a covariance term $-2a\operatorname{Cov}(a,b)/b^3$ would be needed. <Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[Law of propagation of uncertainty and covariance contributions]{ref}
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