For every 20 cakes, 

Blueberry : Mango = 12 : 8 = 3 : 2

For every 5 cakes sold, the collection would be (3 × $28) + (2 × $35) = $154

$3542 ÷ $154 = 23 sets of 5 cakes.

Total number of blueberry cakes sold -> 3 × 23 = 69

Thank you for help🙏🙏