4. In the equation on the right there are five empty squares. Sanja wants to fill four of
6 $\square$
$\square$ $2 \square$ 15 $\square$ $\square 18$ $\square$ $21=45$ them with plus signs and one with a minus sign so that the equation is correct.

Where should she place the minus sign?
A Between 6 and 9
B Between 9 and 12
C Between 12 and 15
D Between 15 and 18
E Between 18 and 21
SolUTION
D

The value of $6+9+12+15+18+21$ is 81 . Now $81-45=36=2 \times 18$. Therefore, Sanja needs to subtract rather than add 18 and hence the minus sign should be placed between 15 and 18 .