3x + 6 = 12
First operation was subtraction (subtracting 6 to leave 3x alone)
Second operation was division (Dividing 3 from 3x to isolare x)
3x + 6 = 12
3x + 6 -(6) = 12 - 6
3x = 6

=

3 and 3 cancels out
x = 2
200 - 8 is 192.
192/4 is 48.
There are 48 students on each train.
Part 1: The simplest way to solve this I think is to set an equation. 4 score is 4x and 7 is +7. So 4x+7=87. Now simply solve.
4x+7=87
-7. -7
4x=80
/4. /4
X=20
---20 years are in a score---
Part 2: start at the year he said it, and go back 87 years. 1863-87=1776.
---he was referring to 1776---
Hope that helped!!