Abig group of people are taking the train home for christmas. 19 people get off the train in brooklyn, 17 people then get on. now there are 63 people on the train. how many were there to begin with?

Let's say that the initial value = i.
So 
i - 19 (because 19 got off) + 17 (because 17 went on) = 63.
From here you just have to do reverse calculations:
i - 19 + 17 = 63
i - 19 = 63 - 17
i = 63 - 17 + 19
Thus, i = 64.This can be understood more easily this way:
i - 19 + 17 = 63
Subtract both sides by 17
soi - 19 + 17 - 17 = 63 -17
Since 17-17 becomes 0:
i - 19 = 63 - 17
Now, add 19 to both sides:
i - 19 + 19 = 63 - 17 + 19
Similarly -19 + 19 = 0
i = 63 - 17 + 19.
Thus once again showing us that there are 65 people initially on the bus

The first answer is b. then the second one is a

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