The answer is $320.
In order to solve this, you can start off by turning this word problem into an expression.
$70x → $70 per hour (x represents the amount of hours)
+40 → plus $40 for the call
Substitute the x for 4 and solve the equation.
70x+40
70(4)+40
280+40
320
The plumber charges $320 for 4 hours of work.