20: Add solutions for section "threads1".

Created by  Aaw9nJhsNmfzFih9mKyNw9mV8CgERXJkRa1kK1Kx3LQH  on August 8, 2021
3PFF4Q4DUKYBG5K4DW4BQ4AA567OWK42TBZLFTN5M7JFLE32QTNQC
In channels
main
Change contents