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%