Question 899885
The problem is peter and renato already finished the job in less than 18 minutes.

a=30
b=40
1/a+1/b=1/x
1/30+1/40=1/x
1/30+1/40=1/17.1428571
1/x=1/17.1428571
x=17.1428571 minutes working together
check
1/a+1/b=1/x
1/30+1/40=1/x
40/1200+30/1200=1/x
70/1200=1/x
1200/70=x
x=17.1428571
ok