If the current debt is 2 trillion dollars, then the GDP must be 2/1.15=$1.7391 X 10^12. If the ratio of 15% must be maintained in 5 years time, then the GDP must grow to: 3/2 X 1.7391 X 10^12
=$2.60865 X 10^12. Because: 3 X 10^12/2.60865 X 10^12=1.15=15%