Eric has got 1 sum wrong

Question

Eric has got 3 sum wrong each time he exactly pressed one wrong key

\begin{align} 5+3+2 & =317 \\ 25+36 &=900 \\ 8+8+2 &=3 \\ \end{align}

can you work out which key he actually pressed

Answer 1

By replacing exactly one character for each sum with another we have: For the equation $5+3+2 = 317$ Eric should type $5 + 312$ (thanks to Andy's kind detection); for the equation $25 + 36 = 900$ Eric should type $25 \times 36$; for the equation $8+8+2 = 3$ Eric should type $8 \div 8 + 2$.

I would say your problem may be stated more clearly such as "For each sum Eric pressed exactly one wrong key", which would make it mathematically precise.

Answer 2

$5+312=317, \quad 25 \cdot 36=900, \quad 8/8+2=3$ so $1,\times,/$ are the right keys.