The static budget, at the beginning of the month, for Wadsworth Company follows:

Static budget:

Sales volume: 2100 units; Sales price: $52.00 per unit

Variable costs: $12.00 per unit; Fixed costs: $26,500 per month

Operating income: $57,500

Actual results, at the end of the month, follows:

Actual results:

Sales volume: 1900 units; Sales price: $58.00 per unit

Variable costs: $17.00 per unit; Fixed cost: $37,000 per month

Operating income: $40,900

Calculate the sales volume variance for operating income. Show all calculations!