I'll call the rate of Jan's garden weeding (fraction of garden weeded per hour) J. Then the rate of her husband's weeding (in terms of fraction of the garden weeded per hour) can be H. With these definitions, there are two things we know: if Jan weeds for 2 hours, the garden is fully weeded; and if her husband weeds for 1.5 hours, the garden is fully weeded. In terms of what we have defined, we can write this mathematically as:
2J = 1 (1 means that 100% of the garden has been weeded, which will happen if Jan works at rate J, defined above, for 2 hours)
1.5H = 1 (husband works for 1.5 hours to fully weed garden)
Now, if both of them work together for some time T, which we are trying to determine, and each works at the same rate they would alone, we know that:
TJ + TH = T(J+H) = 1 (where T is in hours, as the times above are)
From above, we can solve for J = (1/2), and H = 1/1.5 = (2/3). Thus, the most recent equation can be rewritten as:
T((1/2) + (2/3)) = 1 --> T(7/6) = 1 --> T = 6/7 hours
Hope this helps!