Question 992550
.
To average $50 over 4 weeks, 
the sum of his earnings must be:
4*$50=$200
. 
The sum of his earnings the first three weeks was:
$42+$42+$58=$142
.
So for the fourth week he must earn:
$200-$142=$58
.
ANSWER: he must earn $58 the fourth week to average $50 per week.