Problem Solving (PS) | Re: The average of seven numbers is 20. The average of the first four numb

Let the 7 numbers be {a, b, c,d , e, f,g}

The average of seven numbers is 20
We can write: (a + b + c +d + e + f + g)/7 = 20
Multiply both sides by 7 to get: a + b + c +d + e + f + g = 140[equation #1]

The average of the first four numbers is 19 So, (a + b + c +d )/4 = 19
Multiply both sides by 4 to
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