If Sal made a 10% commission on the first $5000, his commission would be 2% more than it is. That is, it would be .02·$5000 = $100 more than it is. In other words, Sal's commission is $100 less than it would be if it were 10% of all sales.
... 10% × $15,000 - 100 = $1500 - 100 = $1400
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The way you're probably expected to figure the commission is by computing the amount subject to each percentage. That is ...
... Sal gets 8% of $5000 = $400
plus
... Sal gets 10% of $15000 - 5000 = 10% of 10000 = $1000
The total of these two amounts is $400 + 1000 = $1400.
