“Dad, we really need a new lawn mower,” Murphy said.

​

 

Of all people, Murphy would know. Since he’d turned twelve, it had been his job to mow the lawn every Saturday morning. The family rule was that it had to be done before any fun activities.

 

 

Murphy had done it so often that he even knew the exact number of times he had to go up and down the yard: 20 times in each direction.

 

 

His father looked at the old lawn mower. It was hard to start, ran rough, and left the grass in clumps. “You’re right. Let’s go to the store and see what they have,” he said.

 

 

At the store, Murphy’s father pointed one out. “Here’s the newest version of the model we have, with the same cutting blade, 60 centimeters across,” he said. “And here’s one with a 75-centimeter blade.”

 

 

Murphy’s little brother, Hugh, who had come along, said, “Here’s one with a 60-centimeter blade, but it’s a mulching mower. It says its blade spins 20% faster than regular mowers.”

 

 

“I think we should buy the one that lets me finish the yard in only 4/5 of the time it takes me now,” Murphy said.

 

 

“Which one is that?” asked Hugh.