If you take a particular two digit number reverse its digits to make a second two digit number and add these two numbers together , their sum will be 121. what is the original number

First number is 10a+b
second number is 10b+a
sum is 10a+10b+a+b=121
11a+11b=121
11(a+b)=121
(a+b)=11

the two digits sum to 11. there are now free enough choices to try them all. the smaller of the 2 numbers must be 29, 38, 47, or 56. of the 4 only 56 ( or 65) can be the original number

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table

The percent increase is 25%