They spent $8 each on cheeseburgers and milkshakes because 3 + 5 = <<3+5=8>>8
They spent $16 on these items in total because 2 x 8 = <<2*8=16>>16
They spent $24 on their meal because 16 + 8 = <<16+8=24>>24
They had $30 to start because 24 / .8 = <<24/.8=30>>30
His cousin had $10 because 30 - 20 = <<30-20=10>>10
#### 10