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
