Quick answer: add up straight-time pay for every hour at its own rate, divide by total hours to get the regular rate, then add half that rate for each overtime hour. 30 h at 18 plus 15 h at 25 is 915 for 45 hours, a regular rate of 20.33, plus 5 x 10.17 = 50.83, so 965.83.
Weighted average overtime
30 hours at 18 plus 15 hours at 25 is 540 + 375 = 915 of straight-time pay for 45 hours. The regular rate is 915 / 45 = 20.33. The 5 overtime hours earn an extra half of that: 0.5 x 20.33 x 5 = 50.83. Total pay 965.83.
For the regular-rate rule behind it, see the overtime calculator; for commissions that change the rate, the commission calculator.
Sources: 29 CFR 778.115; 29 CFR 778.419. Rules checked . Found a problem? Tell us.
Questions
Why only half the rate for overtime hours?
Straight-time pay already includes every hour at 1x. The overtime premium adds the missing 0.5x, so the overtime hours end up at 1.5x in total.