You and your brother each start a child-care service. you spend $50 on supplies and plan to charge $10 per hour. your brother spends $30 on supplies and plans to charge $8 per hour. how many hours will each of you have to work to earn the same amount of money?

You: spends $50 on supplies                              
        charges $10 per hour                                    ⇔
                                                           [The number that 10 & 8 both have in 
Brother: spends $30 on supplies              common is 40]
            charges $8 per hour                                  ⇔
               ↓
You: 10 x 4= 40                [You'll work 4 hours to get the same amount of money.]
                  ↓                                  
Brother: 8 x 5= 40             [Your brother will work 5 hours to get the same amount                                            of money.]