The maturity value is the principal value together with interest due. .. mv = P +Prt .. = 5350*(1 +0.085*120/360) . . . . . . year is 360 days for "ordinary interest" .. ≈ 5501.58
you would ave the denominator to make them both 20 then the fractions would become 5/20 and 8/20 so it would be the son who washed the most. then add them together and get 13/20 so 7/20 laundry needs to be done