Accounting for labour (direct and infirect cost)

Sir, may you help me on below Kaplan exam kit question?
since the extra 3 hours is general overtime and considered as indirect labour cost, why calculate as “40 × $15” and not as “37 X $15” ?

A direct labour employee works a standard 37 hour week and is paid a basic rate of $15 per
hour. Overtime is paid at time and a half. In a week when 40 hours were worked and a
bonus of $20 was paid, what was the direct labour cost?

answer given is $600.
Solution:
40 × $15 = $600. Note that the basic element of overtime is classified as a direct cost. The
normal treatment of overtime premium is for it to be treated as an indirect cost.