To determine the number of hours it takes for sally to paint one house, we use relations from the problem statement above. We use the concept of rates and ratios. For Tom, he would take 10 hrs to finish one house. So, we assign his rate as 1/10. For Sally, she would take x hours to finish one house. So, we assign 1/x. When both of them works, it would take only 4 hours. So, we write the rate as 1/4. It should be that the sum of the individual rates is equal to the rate when both are working.
1/10 + 1/x = 1/4
Solve for x,
x = 6.67 hours