In lte guard band is defined as 10% of available bandwidth.

For 20mhz bandwidth, there is a guard band( narrow frequency range which separates two range of wider frequency)

So 2 mhz is guard band, so total available bandwidth is now 18 mhz.

We know that, 15khz is frequency of sub carrier. For RB calculation there are 12 sub carrier in frequency domain.

Each RB occupy 12x 15khz=180khz ..

so number of RB in 20 mhz band= 18mhz/18khz= 100 RB.