# What is Linear Algebra?

Hi, In the What is Linear Algebra Introduction, I did not get the logic on how they arrived with the formula. Can someone please explain it to me?
Why is 20+d = 50 + a?

What is the link to the exact lesson?

Look at the imageâ€¦there are 4 intersections and four roads. Upper right, upper left, lower left, lower right. They are quadrants on an x, y axis graph. the x axis is horizontal , the y axis is vertical, .

We can break it down on the upper right side of the image:
25+a
30+b

Or, 25 + a = 30+b

Broken down on the upper left side of the image:
20 + d
50 + a
Or, 20+d = 50+a

Lower left:
35+d
40+c

Or, 40 + c = 35 +d

Lower right:

60+b
30+c

60+b = 30+c

1 Like

The idea is that for all these intersections of one-way roads:
the number of cars coming into the intersection is the same as the number of cars coming out out the intersection.
For each intersection: cars in = cars out
The vertical line on the left represents a one-way road where the direction of traffic is â€śupâ€ť.
The horizontal line at the top represents a one-way road where the direction of traffic is â€śrightâ€ť.
For the intersection at the top left,
20 and d are coming in to the intersection,
50 and a are going out from the intersection.
so 20 + d = 50 + a

Maybe the a, b, c, d should have arrows to make that easier to see.