Insert 16 rational numbers between 2.1 and 2.2.

Let:
x = 2.1, y = 2.2 and n = 16

We know:

$d=\frac{y-x}{n+1}=\frac{2.2-2.1}{16+1}=\frac{0.1}{17}=\frac{1}{170}=0.005$ (approx.)

So, 16 rational numbers between 2.1 and 2.2 are:

$(x+d),(x+2 d), \ldots(x+16 d)$

$=[2.1+0.005],[2.1+2(0.005)], \ldots[2.1+16(0.005)]$

= 2.105, 2.11, 2.115, 2.12, 2.125, 2.13, 2.135, 2.14, 2.145, 2.15, 2.155, 2.16, 2.165, 2.17, 2.175 and 2.18