15
8
(7*8)+(7+8) = 56+15 = 71
7+ 7/8=7 and 7/8 or 63/8 Get common denomenators by multiplying 7/1 by 8 to get 56/8. Add 56 and 7 to get 63/8... or 7 and 7/8.
You can, if you have a square with side 5 you can multiply 5*5=25 or you can add 5+5+5+5+5=25 If you want to do this just take one of the numbers and add it to itself the other numbers times. so if you have 7*8 you add 7 to itself 8 times, so add 8 7's 7+7+7+7+7+7+7+7=56 or add 7 8's 8+8+8+8+8+8+8=56 Multiplying is much easier.
You need to add 7 (or +7)
you would add -7.
76
you need to add +9 +9-7 = +2
You need to add 18 to negative 8 to get 10.
21
you need to add 10