If \(a=2\): \(a+20=22\)is not a prime number.
If \(a=3\): \(a+20=23\), \(a+40=43\)are both prime numbers.
If \(a>3\), we have \(a=3k+1\)or \(a=3k+2\).
- \(a=3k+1\Rightarrow a+20=3k+21⋮3\)is not a prime number.
- \(a=3k+2\Rightarrow a+40=3k+42⋮3\)is not a prime number.