Question

How many minutes he does arm routine and abdominal routine

Answer 1

Let $x$ be the amount of time it takes to perform an arm routine and $y$ be the amount of time it takes to perform an abdominal routine.  We see:

$4x+y=70$
$4x+2y=100$

Subtracting the second equation from the first gives $y=30$.  Substituting gives $4x+30=70$, so $4x=40$ and $x=10$.

Thus, an arm routine takes ten minutes and an abdominal routine takes thirty minutes.