If Mrs. Brown earns $3.00 x 40 then she makes $120 for her 40 hours of work. Then she is paid 4 hours of overtime which is worth 1.5 times $3.00 = $4.50
That means that you would take the $120 and add on the overtime $4.50 x 4 = $18, therefore she makes $138 in her 44 hour week.