Chelsea can address graduation invitations in 40 minutes whi

Chelsea can address graduation invitations in 40 minutes, while it would take her mom 1 hour. How long would it take them to address the invitations together?

Solution

Given that

Chelsea address graduation invitations in 40 minutes

Her mother  address graduation invitations in 1 hour or 60 minutes

Chelsea address graduation invitations in 1 minute = 1 / 40

Her mom address graduation invitations in 1 minute = 1 / 60

Both are address graduation invitations in 1 minute = 1/40 +1/60

= (3+2) / 120

= 5 / 120

= 1 / 24

Chelsea and her mom are address the invitations together = 24 minutes