Sennee cut a ribbon into 3 equal pieces . then she cut 5 inches off one piece . that piece is now 3 inches long. what was the length of the original ribbon?

Each piece is x inches long. If she cut 5 inches from x, then it's now x-5 inches long.

This is equal to 3 inches (given) so equate the two expressions and isolate x

x-5 = 3
x-5+5 = 3+5
x = 8

Each piece is 8 inches long. There are 3 pieces. Therefore the original ribbon was 24 inches long (8*3 = 24). 

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38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)