let

x = the hours that jess needs to finish the job alone thus his rate is 1/x houses per hour

y = the hours that jon needs to finish the job alone thus his rate is 1/y houses per hour

after 5 hours of paint work, jess ask jon to help him and they finish the job in 4 hours

5/x + 4(1/x + 1/y) = 1

if jon joined jess after an hour of work they would have finished the job in 7 hours

1/x + 7(1/x + 1/y) = 1

lets substitute a = 1/x and b = 1/y we have the following system of equations

5a + 4(a + b) = 1

a+ 7(a + b) = 1

by solving we find

a = 3/31

b = 1/31

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x = 1/a

x = 31/3

x = 10 h 20 min

y = 1/b

y = 31 h

Jess needs 10 h 20 min ti finish the work alone.