A courier is paid a fixed weekly amount plus the same sum for each parcel delivered. A week with 12 parcels pays £186 and a week with 20 parcels pays £250. What is the fixed weekly amount?

1. Write the two weeks as pairs: 12 parcels with £186, and 20 parcels with £250. 2. Pay changes by £250 minus £186, which is £64. 3. Parcels change by 20 minus 12, which is 8 parcels. 4. Dividing £64 by 8 parcels gives £8 per parcel. 5. In the 12 parcel week, the parcels earned 12 times £8, which is £96. 6. Taking £96 away from the £186 total leaves £90, so the fixed weekly amount is £90. 7. Option C is the £96 earned from parcels, not the fixed amount, which is the trap the working sets up. 8. Options A and D come from using only one week and dividing, which never separates the fixed part from the parcel part. _Source: OpenStax College Algebra (CC BY 4.0), Ch 4 "Linear Functions", section 4.1 Linear Functions_