So we follow the above rule and observe the fourth addition $ 4+5=? $ where we are given operands but not result. We see in the second addition that we have to follow the same rule as for the first and third addition but we have to add 4 in the result. We see in the first and third addition that the second operand is multiplied 10 and added to the first operand.

We now observe alternating additions from top to bottom.

We shall not find any rule that satisfies the first three lines. We try to guess a rule that should work for the first three additions using addition and multiplication.

So multiplication operation is involved in the new rule for addition. We see that the operands in the first three addition $ 1,2 $ or $ 2,3 $ or $ 3,4 $ are very small numbers compared to the result of addition 21, 32, or 43 respectively. We see that the addition operation here is not our usual addition we know.

We are asked to find the missing number in the following puzzle. We find another rule for the second and fourth lines, We find use the rule to find the missing number. We try to guess a rule using the results of newly defined addition here. Hint: We see that the plus sign in this puzzle does not follow the usual addition.

1+2=21 4+5= brain out