how did he found out that 90 was a 30% mark up of 300??????
Let the original price = $100, 30% high means marked price = $130
When marked price = $130 original price = $100
When marked price = $1 original price = $100/130
When marked price = $390 original price = $(100/130) x 390 = $300
Now he has to sell it for 20% less. 20% of 300 = 20 x 300 x 1/100 = $60
So he will get $240 for cellphone