18. The sum $3+5 \times 7-9=36$ is incorrect. However, if one of the numbers is increased by 1 , it becomes a correct calculation. Which number should be increased?
A 3
B 5
C 7
D 9
E 36
Solution
B

The value of $3+5 \times 7-9$ is 29 . If this value is increased by 7 , it would equal the value on the right-hand side of the original equation. Hence, if we increase the number of 7 s on the left-hand side by 1 , the two sides will be equal. Therefore, the number which should be increased by 1 is 5.// It is left to the reader to check that increasing any other number by 1 will not produce a correct equation.