One eraser costs 20 cents.
Let p = the amount of pencils and e = amount of erasers.
Equation 1. 6p + 5e = $1.60
Equation 2. 3p + 3e = $0.90
If we times equation 2 by 2 to get 6p + 6e = $1.80 and call it equation 3 we can minus equation 1 from it to get equation 4.
Equation 3. 6p + 6e = $1.80
Equation 1. - ( 6p + 5e = $1.60 )
Equation 4. e = $0.20
Therefore an eraser is 20 cents.
Hope that helps.
