A newsagent sold 100 copies of a magazine in 8 dayseach day he sold 3 more copies than on the previous dayhow many did he sell on day 4

On day 1 he sold x
On day 2 he sold x + 3
On day 3 he sold x + 6
On day 4 he sold x + 9
On day 5 he sold x + 12
On day 6 he sold x + 15
On day 7 he sold x + 18
On day 8 he sold x + 21
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Adding these together
After day 8 he had sold 8x + 84
8x + 84 = 100
8x = 100 - 84
8x = 16
x = 2

So on day 4 he sold 2 + 9 = 11

Day 1 = 02 - total 002
Day 2 = 05 - total 007
Day 3 = 08 - total 015
Day 4 = 11 - total 026
Day 5 = 14 - total 040
Day 6 = 17 - total 057
Day 7 = 20 - total 077
Day 8 = 23 - total 100