Pay-for-performance I

A father decides to put his two sons to work landscaping. The business involves going to a customers home and providing landscaping services (cut grass, edge sidewalk, pull weeds in flower beds, prune bushes and trees, rake leaves). Instead of paying a flat wage, the father decides to pay an incentive according to the following schedule (average across all lawns).

Task	Piece rate incentive per person	Physical effort	Time to complete the task	Charge to customer
Cut grass	$40	Easy	25 minutes	$300
Edge sidewalk	$10	Easy	5 minutes	$50
Pull weeds	$60	Very hard	30 minutes	$400
Prune bushes	$50	Hard	30 minutes	$300
Rake leaves	$50	Hard	30 minutes	$250

At the end of the second week, the boys are quarrelling with each other and not happy with their dad. All of the disagreements revolve around the incentive system. What might be the problems?