gross annual income: 39,600
gross monthly income: 39,600 / 12 = 3,300
a) maximum amount for monthly mortgage payment: 3,300 x 28% = 924
b) maximum amount for total credit obligations: 3,300 x 36% = 1,188
c) mortgage: 924 x 70% = 646.80 actual mortgage
1,188 - 646.80 = 541.20 maximum amount they could spend each month for all other debts.