tinayni552 wrote:
tinayni552 wrote:
IanStewart wrote:
I think you're arriving at an incorrect conclusion because the equation I highlighted above is not right - it should read 20 + b = d.
You can write down four equations here in four unknowns, but the equations are not "independent", to use the technical term. You can derive one of the equations from the rest of them. If you take the first three equations and put the number alone on one side in each:
10 = a + b
20 = d - b
30 = c - a
and just add all three of those equations together,
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