Start with what the question "how many students were in each bus?". We dont know that value, so let's call that x for now.
x = number of students in 1 bus
We have 3 buses, so we represent that by 3x
3x = number of students in 3 buses
Then add on 4 to represent the number of students who took a car
3x+4 = total number of students
3x+4 = 124 is the equation we need to solve
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Let's solve for x
3x+4 = 124
3x+4-4 = 124-4 .... subtract 4 from both sides
3x = 120
3x/3 = 120/3 .... divide both sides by 3
x = 40 is the number of students in one bus
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Each bus holds 40 students
3 buses hold 3*x = 3*40 = 120 students
adding on those 4 additional students gets us to 120+4 = 124 so we've confirmed the answer.