The basis for computing the compensation provided for in Sections 7 and 8 of the Act shall be as follows:
     The compensation shall be computed on the basis of the “Average weekly wage” which shall mean the actual earnings of the employee in the employment in which he was working at the time of the injury during the period of 52 weeks ending with the last day of the employee’s last full pay period immediately preceding the date of injury, illness or disablement excluding overtime, and bonus divided by 52; but if the injured employee lost 5 or more calendar days during such period, whether or not in the same week, then the earnings for the remainder of such 52 weeks shall be divided by the number of weeks and parts thereof remaining after the time so lost has been deducted. Where the employment prior to the injury extended over a period of less than 52 weeks, the method of dividing the earnings during that period by the number of weeks and parts thereof during which the employee actually earned wages shall be followed. Where by reason of the shortness of the time during which the employee has been in the employment of his employer or of the casual nature or terms of the employment, it is impractical to compute the average weekly wages as above defined, regard shall be had to the average weekly amount which during the 52 weeks previous to the injury, illness or disablement was being or would have been earned by a person in the same grade employed at the same work for each of such 52 weeks for the same number of hours per week by the same employer. In the case of volunteer firemen, police and civil defense members or trainees, the income benefits shall be based on the average weekly wage in their regular employment. When the employee is working concurrently with two or more employers and the respondent employer has knowledge of such employment prior to the injury, his wages from all such employers shall be considered as if earned from the employer liable for compensation.