Can you explain how the expert got the expected percentage?

Class Lower limit - Upper limit f; xbar # 1s xbar + 2s xbar + 3s 20 29 9 9 30 I 39 23 0.92 23 40 49 32 5.76 32 32 50 59 75 75 75 75 60 69 90 90 90 90 70 79 77 77 77 77 80 1 89 34 2.72 34 34 90 99 19 14.44 19 100 1 109 6 6 Total 365 250.48 323.36 365 Last step, to determine percentages: Percentage for mean plus minus one SD: 250.48 P = 365 . 100% = 68.62 % Final table: xbar + 1s xbar + 2s xbar + 3s Days 250.48 323.36 365 Percentage of year 68.62 % 88.59 % 100.00 % Expected percentage 68.3 % 95.4% 99.7%