3 Marker + 9 Magnet= $10.20
Money left: If buy
Marker short $0.80
Magnet left $0.40
Hence, we know 1 Marker is ($0.80+$0.40) = $1.20 more then 1 magnet.
From 3 Marker + 9 Magnet= $10.20
(3 marker = 3 magnets + 3 x $1.20 = 3 magnets + $3.60)
(3 magnets + $3.60) + 9 magnets = $10.20
12 magnets = $10.20 - $3.60
12 magnets = $6.60
1 magnet = $0.55 [6.60 ÷ 12 = 0.55]

a) Whiteboard marker = $0.55 + $1.20 = $1.75
b) Money at 1st = $10.20 + 1 magnet + $0.40
= 10.20 + 0.55 + 0.40
= $11.15

Hope this helps.