We can try as follows:
\(400\) is \(8\cdot 50\) and \(600\) is \(8\cdot 75\)
Average of \(400\) and \(600= 500\), extreme multiples of \(8\)
Number of multiples of \(8=75-50+1=26\)
Sum \(= \text{Number of multiples} \cdot \text{average}= 26 \cdot 500=13000\)
