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