Instrumentation Amplifier - circuit analyze
Search for "Bias Current Compensation" and you'll find a useful paper from Analog Devices that covers the basics. What follows is a somewhat hand-wavy explanation.

Unlike many opamps, the LT1013/14 does not have internal input bias current compensation. The input transistors are PNP, so all of the inputs function as current sources, with values on the order of 19 nA.

The fourth opamp in the diagram above (pins 12, 13 and 14) is used for bias current compensation for the two opamps directly above it, in order to eliminate the need for the source circuit to deal with it. It provides a sink for the bias currents of pins 3, 5, 12 and 13, reducing the residual bias current seen by the driving circuit to &lt;1 nA.

Negative feedback means that pin 13 has the same voltage as pin 12, and therefore, the bias current flowing through pins 12 and 13 is the same. It is also the same as the current from pin 5, and if there is zero differential input, the current from pin 3 as well. The voltage at pin 14 is whatever it takes to make the current through the "2R" resistor attached to pin 13 equal to the bias current. The current through the "2R" resistor attached to pin 3 is the same. There are 2 inputs (twice the bias current) attached to the resistor connected to pins 5 and 12, so the resistor must be half the value, or just "R", in order for it to have the same voltage drop.