Question 1011200
Well if he spends 20% on rent and 70% of what's left (which is 80%) on household expenditures, then what he has left is 
100% - 20% - (.70)(80%) = 24%
So 24% of his income is Rs 1800, thus we write
.24x = 1800
x = 1800/.24
x = Rs 7500