A: divide annual salary by 12 months:
42,000 / 12 = $3,500 per month
B: multiply sales by the percentages:
First 40,000 x 2.5% = 40,000 x 0.025 = $1,000
Amount over 40,000 = 59,000 - 40,000 = 19,000 x 4% = 19,000 x 0.04 = $760
Total commission = 1,000 + 760 = $1,760
c. Add a and b:
3,500 + 1,760 = $5,260