Päättele aloitusluku.

a. [[$\fbox{ +13 } \fbox{ –69 }$]] = 91

b. [[$\fbox{ · 2 } \fbox{ : 4 } \fbox{ –10 }$]] = 10

c. [[$\fbox{ + 50 } \fbox{ : 10} \fbox{ –35 } \fbox{ · 3 }$]] = 15

d. [[$\fbox{ : 6 } \fbox{ + 70 } \fbox{ + 45 } \fbox{ –55 }$]] = 85