If a positive number is increased by 70% and then the result is decreased by 50% what is the overall percentage change

Im not one hundred percent sure... but im pretty sure its overall percentage change is 120%

Let's pick a number, lets say 100
increase by 70% means add 70
end result we get 170
then decrease by 50% or half of 170
170 -(170/2)=85

comare end and beginning
100 resulted to 85
so we end with 85% of the original
change=origianl-final=100-85=15

change is 15%

This does not make sense