Check your 555 circuit theory with practice:
555 - Astable
  Enter values for your breadboard / circuit
  R1 =  K  
  R2 =  K  
  C1 =  μF  
  Calculate the astable frequency (f) to 3 sig figs
Your astable (pulse output) frequency (f) =  Hz

For more detail look here
The formula is in the specification. Also, click on 'Help'.
It will be given in the examination.
You will need a calculator!
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