She uses 2 pencils a day because 10 / 5 = <<10/5=2>>2
She will use 90 pencils in 45 days because 2 x 45 = <<2*45=90>>90
She will need 3 packs of pencils because 90 / 30 = <<90/30=3>>3
She will spend $12 because 3 x 4 = <<3*4=12>>12
#### 12