Frank worked 25 hours as a waiter last week.
If Frank worked both jobs for a total of 36 hours, hence:
x + y = 36
x = 36 - y
Also, if he earns $12 per hour as a waiter and $10 per hour as a busser and earned a total of $410, then;
12x + 10y = 410
Substitute 1 into 2
12x + 10y = 410
12(36 - y)+ 10y = 410
432 -12y + 10y = 410
-2y = -22
y = 11
Since x = 36 - y
x = 36 - 11
x = 25
Hence frank worked 25 hours as a waiter last week.
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