
Confidence Interval
Concentrations of atmospheric pollutants such as carbon monoxide (CO) can be measured with a spectrophotometer. In a calibration test, 50 measurements were taken of a laboratory gas sample that is known to have a CO concentration of 70 parts per million ( ppm). A measurement is considered to be satisfactory if it is within 5 ppm of the true concentration. Of the 50 measurements, 37 were satisfactory.
a. Find a 95% confidence interval for the proportion of measurements made by this instrument that will be satisfactory.
b. How many measurements must be taken to specify the proportion of satisfactory measurements to within ± 0.10 with 95% confidence?
without the st. dev and mean i don't know how to go about this problem
confidence intervals are the bain of my existence

Confidence interval will be
$\displaystyle \displaystyle p\pm Z_{1\frac{\alpha }{2}}\times \sqrt{\frac{p(1p)}{n}}}$