b + e = constant => a+ c is constant.

All you need to fill in is the arithmetic mean. Do you know what that is?

Bob

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I am not very clear about what is variable and what fixed here. Let's see if I can sort this out.

Let beginning = b

end = e

first quantity = a

second quantity = c

equidistant => a - b = e - c

=> a + c = e + b

So, if e and b are fixed it should be easy for you to finish this off.

Bob

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