87. Solving a chemistry problem

4. Determine the order of the reaction 3A(gas)→A3(s). The pressure of the initial substance in one experiment dropped from 0.252∙105 to 0.205∙105 Pa in 31 hours, and in another experiment, which was carried out in the same vessel at the same temperature, from 0.105∙105 to 0.102∙105 Pa in 20 hours.