More answer number 14, telling me how to set up the problem and the table

Part a) 
we know that
it takes 7 hours for Isabella to rake leaves
so
7 hours---------------> 100%
1 hours---------> x
x=100/7-----> 14.29% job/hour--------> this is the rate  or 0.1429 job/hour

Matthew can work three times as fast
so
14.29*3-----> 42.87% job/hour-------> this is the rate or 0.4287 job/hour

if they work together
(14.29%+42.87%)=57.16%/hour

57.16%-------------> 1 hour
100%---------> x
x=100/57.16--------> x=1.75 hours-------> x=1 3/4 hours

the answer Part a) is
1 3/4  hours

Part b) complete the table  

see the attached figure