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Ok we'll take an example with a 30 year old and write it out mathematically
(2x + 5)50 + 1755 - 1975 = 100x + 30
we get the 100x because it will be a three digit number with the first number (the hundreds) as x. So for 2 chocholates it'll be: 200 + your age (30). For 3: 300 + 30 and so on.
Now we expand as zen_tom has done above and obtain
100x + 250 + 1755 - 1975 = 100x + 30
100x + 30 = 100x + 30
x = x
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