Question 1036895

I am a number between 70 and 80. The second digit is 3 smaller then the first digit. I know it's 74 because 7-4=3, but how can i solve it mathematically?
<pre>Let the tens and units digits be T and U, respectively
Then the number is: 10T + U 
Since it's between 70 and 80, T MUST be 7, and so the number is: 10(7) + U, or 70 + U
Also, it's given that the second (units) digit is 3 smaller than the first (tens) digit, so we can say that: U = T - 3, or U = 7 - 3, or 4
With the tens digit being 7 and the units digit, 4, the number's: {{{highlight_green(74)}}}