nominal GDP year 1=2*5+3*6=28
nominal GDP year 2= 4*9+6*8=84
Notice that you can not compare production between two years because prices increased. It is not correct to state that production multiply by 3. So you should compute real GNP at prices of year 1
Real GDP year 1 (at prices of year 1) = 28 (same as before)
Real GDP year 2 (at prices of year 1) = 4*5+6*6=56
So you are correct if you say that real GNP doubles
Nominal GNP vs Real GNP:
GNP is a measure of the economic output of an economy. Real and nominal
GNP are both used for comparisons between different economies, but they
approach the comparison in different ways.
If prices rise, then the nominal GNP will look like it increases even
if it doesn't actually increase. This is because the total output in price will
increase. Real GNP, therefore, can help adjust for inflation and rising costs
while still providing an accurate measure of the total economic output. Real
GNP can be used for measuring economic output in terms of goods and services,
while nominal GNP can be used to measure output in terms of money value.