Solution 1 Let the single digit of the original two-digit number be X and the decimal number be 9-X..

Then the original two digits are10 (9-x)+X.

After exchanging the number in one number with the number in ten numbers

The two digits of are10x+9-x.

From the title,10x+9-x =10 (9-x)+x+9.

The solution is x=5.

There are X toys in the nail box.

Then box b has x+40.

Then x+40+7=3(x+5)

The solution is x= 16.

If there are x chickens, there are 100-x rabbits.

Then 2x-4( 100-x)=80.

The solution is x=80.