He needs 18 valentine's cards because 30 x .6 = <<30*.6=18>>18
He will spend $36 on these because 18 x 2 = <<18*2=36>>36
The proportion of his income spent is .9 because 36 / 40 = <<36/40=.9>>.9
This is 90% of his income because .9 x 100 = <<.9*100=90>>90
#### 90