Gingerbread house

Janka and Marienka calculated that there are 210 gingerbreads on the gingerbread house. Janko ate one-seventh of all gingerbreads, and Marienka ate a third less than Janko. How many gingerbreads remained in the gingerbread house?

Correct result:

n =  160

Solution:

$$\begin{gathered} J=210\mathrm{/}7=30 \\ M=(1-1\mathrm{/}3)\cdot J=(1-1\mathrm{/}3)\cdot 30=20 \\ n=210-J-M=210-30-20=160 \end{gathered}$$

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