5-1=        10-2=        18-1=        8+3=

2+4=        6+9=        12-2=        14-1=

7+5=        9+7=        5+8=        9-7=

3+7=        10+3=       10-3=       8+7=

13－2{#blank#}1{#/blank#}12                20－1{#blank#}2{#/blank#}19＋0

19{#blank#}3{#/blank#}7＋6                   4＋9{#blank#}4{#/blank#}15－4

10-{#blank#}1{#/blank#}=4             2+{#blank#}2{#/blank#}=7         {#blank#}3{#/blank#}+10=18

{#blank#}4{#/blank#}+3=9            {#blank#}5{#/blank#}-4=5            8-{#blank#}6{#/blank#}=3

他们一共借了多少本书？