At the beginning of 2020, the population of three towns – Alpha, Beta, and Gamma – was in the ratio 3:5:7, respectively. During the year 2020, the population increased by percentages in the ratio of 2:3:1 for the respective towns. By the beginning of 2021, the population of Gamma was 7000 more than that at the beginning of 2020, and by the start of 2021, the ratio of the population of Alpha to Beta was 36:65. What was the total increase in the population of all three towns during the year 2020?