Maths challenge question

Despite his name, Mr. Bean likes to eat lots of fruit. He finds that four apples and two oranges cost £1.54 and that two oranges and four bananas cost £1.70. How much would he have to pay if he bought one apple, one orange and one banana?

I know its 30 34 and 17 to give 81p but I knid of got that by trial and error.
I have
4a + 2o = 1.54
4b + 2o = 1.70
so b-a = 0.04
Now what?