A lots of students, maybe including instructor, is not aware of the design of instrumentation amplifier. First let’s take a look at the schematics:
I am not intested in deriving the gain formula but need it here anyway:
I tried to address here is the two parts of the gain: (1+2R1/Rgain) and (R3/R2).
I have seen student design with R3=R2 so that the second differential amplifier has a gain of 1 and then he uses the first gain formula to calculate for a gain of 25. The circuit did not work because he is using 15V or even 12V which is easily saturated with the gain of 25 when V1 or V2 is more than 1V. The difference of V2-V1 can be small to be amplified but because of the gain distribution does not permit a proper gain increase, the formula is equivalent to garbage when come to using it.
This is why I strongly against any student tries to use formula without understanding the logic behind.