Answer:
20
Explanation:
For the sake of the problem, let's make female workers "x" and male workers "y". 
 
x+y<40 This equation shows that the total number of workers has a max of 40. 
 
30x+20y<1,000 This equation shows that the total cost the manager pays ($30 to each woman, $20 to each man) has a max of $1,000. 
 
Now you can solve for x and y. 
 
X+y<40
-y -y
X<-y+40 
 
Substitute -y+40 in for X in the second equation 
 
30(-y+40)+20y<1,000 
-30y+1200+20y<1,000 Distribute
-10y+1,200<1,000 Combine like terms
-10y<-200 Subtract 1,200
y>20 Divide by -10; flip the sign 
 
 
Since y>20, and y=male workers, you now know that the minimum
number of male workers he should send is 20