A salesman's commission varies directly as his sales.If the commission is $10 for $100 in sales, find the commission for $150 in sales.

A direct variation means the commission-to-sales ratio stays constant. The rate is $10 ÷ $100 = $0.10 per dollar of sales, so for $150 the commission is $0.10 × $150 = $15. The correct choice is $15.

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