What is the capacitance of a 20-microfarad capacitor connected in series with a 50-microfarad capacitor?
Why Capacitors in series combine like resistors in parallel: 1/Ct = 1/C1 + 1/C2, or for two capacitors the shortcut Ct = (C1 x C2)/(C1 + C2). Here that is (20 x 50)/(20 + 50) = 1000/70 = 14.3 microfarads. Note the result is smaller than either capacitor, which is always true for series capacitors because you are effectively increasing the plate spacing.
Watch out The 70 microfarad choice is just the sum, which is what you would get if the capacitors were in parallel; 1,000 microfarads is only the numerator of the product-over-sum formula, and 0.07 is the reciprocal of the answer.
Series caps: product over sum, and the answer must be SMALLER than the smallest capacitor.