Question 1199553
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Once a month, Benfort put some money into the bamboo jar. Each month, he put 50 centavos more 
into the jar than the month before.  {{{highlight(At_the_end_of)}}} {{{highlight(cross(On))}}}  its 12th yr, 
he counted his money for his graduation; and he had Php 5, 436. How much money did he put in the jar in the last month?
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            There is an error in calculations by @greenestamps,

            So I came to fix this deficiency.

            The place where I changed the @greenestamps' numbers, is marked in red in my post.

<br>
Once a month for 12 years means 12*12 = 144 deposits.<br>
The final amount was 5436; the average deposit was 5436/144 = 37.75.<br>
The average deposit is the average of the first and last deposits:
(first + last)/2 = 37.75
first + last = 2*37.75 = 75.5  [1]<br>
The last deposit is the first deposit, plus the common difference of 50c = 0.5p &nbsp;&nbsp;{{{highlight(cross(119))}}} &nbsp;&nbsp;{{{highlight(highlight(143))}}} &nbsp;&nbsp;times:
last = first + 143(0.5) = first + 71.5  [2]    &nbsp;&nbsp;&nbsp;&nbsp;<<<---  starting from this point, the numbers are different from that of @greenestamps<br>
From [1] and [2]...
first + last = first + first + 71.5 = 75.5
2*first = 4
first = 2  [3]<br>
From [2] and [3], last = 2 + 71.5 = 73.5<br>
ANSWER: He put Php 73.5 in the jar the last month<br>
CHECK: {{{144((2+73.5)/2)=144*37.75=5436}}}<br>


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Notice that I edited you post to make the meaning absolutely clear,
as it &nbsp;SHOULD &nbsp;BE &nbsp;in any &nbsp;Math problem.