7. Mental arithmetic. Square 85 in your mind.
7. Since $(10 a+5)^{2}=100 a^{2}+100 a+25=a(a+1) 100+25$, we get that $(85)^{2}=8 \cdot 9 \cdot 100+25=7225 \star$.

$$
[\text { M. M., 24, } 273 \text { (May 1951).] }
$$