What i understand is 3/5 of the balls are yellow and all of these will be sold as 3 per can.
If so,
3/5 of 4250 tennis balls is 2550 (4250*3/5=2550)
These are yellow tennis balls.
If you pack as 3 of these balls, you get 850 cans. (2550/3 =850)
And if you sell these as £2.50 per each, 850*£2.5=£2125
85% of this money goes to charity;
£2125 * 85/100 = £1806.25