RB RE Speed

Resource Element:
Covered by 1 sub carrier and one symbol period i.e 1 Symbol

Resource Block:
Covered by 12 Subcarriers and 6 or 7 symbols(Based on Cyclic prefix)

We can calculate the capacity if we know the Bandwidth allocated,Modulation scheme used and Cyclic Prefix type.
Consider 
Bandwidth Allocated = 5MHz
Modulation Scheme = QPSK(2 its per symbol)
Cyclic prefix used = Normal Cyclic Prefix (7 symbols in a slot)

Capacity = No of bits transferred in a sub frame / Duration of the Subframe(i.e 1milli sec)

Calculation No of Bits transferred in a Subframe

Total number of Resource block in a slot = Bandwidth / (Each subcarrier bandwidth * no of subcarriers in a Resource Block) 
Bandwidth = 5Mhz
Each sub carrier width = 15Khz
No of sub carriers in a Resource Block = 12 (As 12 Subcarrier is 1 Resource block )

Calculation of total no of resource blocks in 5MHz Bandwidth

so Resource block in one slot = 5 Mhz / (15 KHz *12 ) = 27.77 (But used RBs will be 25 for 5MHz bandwidth)
so Total Resource block in one subframe = 2 * Resource blocks in a slot = 50 RBs

Calculation of total no of symbols/Resource Elements in 5MHz Bandwidth
Total no of symbols / Resource Elements = 50 * no of symbols(REs) in a Resource block
No of symbols(REs) in a resource block = 12 subcarrier * 7 symbols = 84 symbols(REs)
Total symbols(REs ) in 50 Resource blocks = 50 * 84 = 4200 Resource Elements.

****Calculation of total no of bits sent in 5MHz Bandwidth in one subframe ****

In one symbol we can send no of bits at a time which depends on the modulation scheme (like BPSK,QPSK ,64QAM
BPSK - 1 bit,QPSK - 2 bit ,64 QAM - 6bits in a Symbol/ Resource elements
We will consider QPSK Scheme so in one symbol 2bits we can send.
So 1 Symbol = 2 bits.

So total no of Bits sent in a subframe = 2 * 4200 = 8400 bits.

So capacity = 8400 / 1ms = 8.4 MBPS

So capacity in 5MHz Bandwidth with QPSK Modulation with normal cyclic Prefix is 8.4MBPS.

if Modulation scheme is BPSK, 1 bits in a symbol = 1 * 4200 / 1ms = 4.2 MBPS
if Modulation scheme is 64QAM , 6 bits in a symbol = 6 * 4200 / 1ms = 25.2 MBPS.