Question 937783
The sum of a hundreds digit and the tens digit of a three digit number is 13. If the hundreds digit and the tens digit are interchanged, the new number is 90 less than the original number. It is given that the units digit of the original number is 4. Find the original number. 
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let u=units digit
let t=tens digit
let h=hundredth digit
t+h=13
t=13-h
u=4
..
100h+10t+4-(100t+10h+4)=90
100h+10(13-h)+4-(100(13-h)+10h+4)=90
100h+130-10h+4-1300+100h-10h-4=90
200h-20h-1170=90
180h=1260
h=1260/180=7
t=13-h=6
original number: 764
..
Check: Interchange hundredth and tens digit
original-new number=764-674=90