Question 313393
Martin ate 100 cookies in five days. Each day he ate 6 more than the day before. How many cookies did he eat on the first day?


Let the amount of cookies that Martin ate on the 1st day be F


Since he ate 6 more cookies than the previous day, then he would've eaten: 


(F + 6) cookies on the 2nd day
F + 6 + 6, or (F + 12) cookies on the 3rd day
F + 12 + 6, or (F + 18) cookies on the 4th day
F + 18 + 6, or (F + 24) cookies on the 5th day


Since he had eaten a total of 100 cookies in 5 days, then we have:


F + (F + 6) + (F + 12) + (F  + 18) + (F + 24) = 100


5F + 60 = 100


5F = 40

F = {{{40/5}}} = 8


Therefore, Martin ate {{{highlight_green(8)}}} cookies on the 1st day.

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Check
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Day 1: 8
Day 2: 8 + 6 = 14
Day 3: 14 + 6 = 20
Day 4: 20 + 6 = 26
Day 5: 26 + 6 = 32


In 5 days, he ate: 8 + 14 + 20 + 26 + 32 = 100