They have been saving their money for 10 weeks.
Duane = $5 every two weeks
Mick = $2 every week.
Get the value of Duane's weekly contribution = $5 / 2 = $2.5
Duane's share + Mick's share = $45
$2.5x + $2.0x = $45 ===>> equation can also be x(2.5 + 2) = 45
$4.5x = $45
$4.5x / $4.5 = $45 / $4.5
x = 10 weeks.
To check:
$2.5(10) + $2(10) = $45
$25 + $20 = $45
$45 = $45