Maci pays 10 * $0.10 = $<<10*0.10=1>>1 for the blue pens.
Each red pen costs 2*$0.10 = $<<2*0.10=0.20>>0.20
She pays 15*$0.20 = $<<15*0.20=3.00>>3.00 for the red pens.
To buy all the pens she needs, Maci pays $3+$1= $<<3+1=4>>4
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