Let

x = the weight that Jeremy can lift

y = the weight that Jeremy's father can lift

x = y + 65 (Jeremy can lift 65 pounds more than his father)

x + y >=405 (together they can lift at least 405 pounds)

That is

y + 65 + y >=405

y >= 170

Then x = y + 65 >= 170 + 65 that is x >= 135.

Jeremy's father can lift at least 170 pounds and if x is the weight that Jeremy can lift then y = x - 65.