Answer
$T = 71.7~N$
Work Step by Step
The tension in the cable pulling up on the bottom pulley is $2F$
The tension in the cable pulling up on the middle pulley is $4F$
The tension in the cable pulling up on the top pulley is $8F$. Then $T = 8F$
The forces pulling up on the block are $F, 2F,$ and $4F$
We can find the value of $F$:
$F+2F+4F = mg$
$7F = mg$
$F = \frac{mg}{7}$
$F = \frac{(6.40~kg)(9.8~m/s^2)}{7}$
$F = 8.96~N$
We can find $T$:
$T = 8F$
$T = (8)(8.96~N)$
$T = 71.7~N$
