In 1.5 you must farm 100 exp, in 2.0 you must farm 200 exp till level. If you had 64% = 64 exp in 1.5. In 2.0 you will have 64 exp = 32%. Right? You had to farm 2 times exp more so you will lose half of the %. 200exp/64=3.1, then you need to 100% of level / 3.1, mean 100%/3.1 = 32% you will lose 32%
Your friend had 12 exp = 12% in 1.5. In 2.0 he will have 12 exp = 6%. Again: 200exp/12= 16.6, 100%/16.6=6%, he will lose 6%.
Ask your self how much % is 50 exp when till level you need 100 exp, and how much % is 50 exp if till level you need 200 exp. 50% and 25% ( hint )
Then as your self how much % is 10 exp when till level you need 100 exp, and how much % is 10 exp if till level you need 200 exp. 10% and 5% ( hint )