From the last two facts, we get
[tex]\dfrac{x_1+x_2+x_3}3=50\implies\dfrac{x_1+x_2+x_3}7=\dfrac{150}7[/tex]
[tex]\dfrac{x_5+x_6+x_7}3=70\implies\dfrac{x_5+x_6+x_7}7=30[/tex]
Then
[tex]\dfrac{x_1+x_2+\cdots+x_6+x_7}7=60\implies\dfrac{x_4}7=60-\dfrac{150}7-30\implies\boxed{x_4=60}[/tex]
