Let the third number is x.
then first number = (100-60)% of x
= 40% of x = 4x/10
Second number is (26x/100)
Difference = 4x/10 - 26x/100 = 14x/10
So required percentage is, difference is what percent of first number
=> (14x/100 * 10/4x * 100 )% = 35%