a competitive labour market, the individual firm is not big enough to influence the wage rate. The marginal cost of labour is a horizontal line at the existing market wage rate.
MCL
5.50
20
7.00
Employing the 20th unit of labour costs the firm £5.50 per hour but that labour adds £7.00 per hour to total revenue through their work. It is worth employing that extra unit of labour.
6.70
21
The 21st unit of labour adds slightly less to total revenue (£6.70) but still costs £5.50 and so is worth employing. There will thus be an incentive for the firm to continue to employ additional units of labour until the MRP = Wage rate
For the employer to be persuaded to employ additional workers, therefore, the wage rate must be lower to compensate for the fact that the extra worker adds less to total revenue than the previous one and to sell extra units, the firm must accept a lower price.