Excellent work Maaverix & Flanostu

In 7*5 = 35 days, Jack can paint 7 houses.

In 5*7 = 35 days, Richard can paint 5 houses.

So in 35 days, the two of them can paint 12 houses. To paint just one house, they'll need 1/12 the time, or 35/12 = 2 11/12 days.

So =35/12 =2.92 Days