Maths problem sum help

Ian takes 7 hours more than Joseph to complete an assignment alone. If both Ian and Joseph can take 5 hours to complete the same assignment together, how much time, in hours, will Joseph take to complete the same assignment alone? Correct your answer to 3 significant figures.

would appreciate working, tyvm. :)

Together, they complete the job in 5 hours. That means that every hour, they complete 1/5 of the job

1/I + 1/J = 1/5
(J + I) / (I * J) = 1/5
5 * (J + I) = I * J
5J + 5I = IJ

I = J + 7

5J + 5 * (J + 7) = (J + 7) * J
5J + 5J + 35 = J^2 + 7J
10J + 35 = J^2 + 7J
J^2 - 3J - 35 = 0
J = (3 +/- sqrt(9 + 140)) / 2
J = (3 +/- sqrt(149)) / 2
J = (3 + sqrt(149)) / 2
J = 7.6032778078668514759489276283115
J = 7.603

I = J + 7
I = 14.603

1/(7.603) + 1/(14.603)

might not be right...

I = J +7
(I + J)/2 =5
solve for J..

(2J+7)/2 =5
2J + 7 = 10
2J=3
J=1.50