Question

Dan the detective looked for a number. he found a two-digit number less than 50. the sum of its digits was 12. their different was 4. what number did da find

Answer 1

You can quickly tell it is 48, that's the only possibility
for numbers that you cannot quickly tell, suppose the one's digit is x, and the tens' digit is y
in this case, x+y=12 and x-y=4. 
add the two equation to eliminate y: 2x=16, x=8
y=x-4=4
the number is 48