<———-$1——->< 50c>< ———-20c——–>

[             ][             ][            ][            ][             ]

[u][u ][u] [u][u ][u] [u][u ][u] [u][u ][u][u][u ][u]

<——-6u———> <-2u-><——-7u————–>

$1                  [     $       ][97.20]

50c + 20c     [     $       ]

(a)  5 units total x 3 = 15 units  (to show that $1 stamps is 3 times as many as 50c stamps)

       Hence fraction of 20c stamps = 7u/15u = 7/15

(b) 6u(1) – 2(0.5) – 7(0.2) = 97.20

        6u – u – 1.4u = 97.2

          3.6u = 97.2   => u = 27

Total number of stamps sold by store = 15u = 15 x 27 = 405

PS: Please click “Accept Answer” if the answer is correct.