Khairi only has twenty cent and fifty cent coins in his coin bank. There are twice as many twenty cent coins as fifty cent coins. If all the coins add up to $17.10, how many twenty cent coins are there in the coin bank?
Let's say Khairi has x fifty cent coins.
The total value of the fifty cent coins is x*50 = <<50*x=50x>>50x cents.
If there are twice as many twenty cent coins as fifty cent coins, then the number of twenty cent coins is 2*x = 2x.
The total value of the twenty cent coins is 20*2x = 40x.
The total value of all the coins is 40x + 50x = 90x
The total value of all the coins is also 1710 cents, so 90x = 1710.
Solving the equation above, we find that x = 1710/90 = <<1710/90=19>>19.
Therefore, the number of twenty cent coins is 2x = 2*19 = <<2*19=38>>38. Answer: \boxed{38}.