Martin, Tomáš, and Jirka guessed the result of the expression:

$$
2.4 - 1.5 \cdot 3.6 : 2 =
$$

Their average guess differed from the correct result by 0.4. Tomáš guessed -1.2 and Jirka 1.7. Determine which number Martin guessed, given that the worst guess differed from the correct result by 2.
Let Martin's tip be $m$. The correct result is $2.4 - 1.5 \cdot 3.6 : 2 = -0.3$. Since the worst tip differs from the correct result by 2, Jirka had the worst tip $(1.7 - (-0.3) = 2)$. The average tip of the boys differed from the correct result by 0.4. Therefore, the average tip was either $-0.7$ or 0.1.

a) The average tip is $-0.7$. Then we have

$$
\frac{-1.2 + 1.7 + m}{3} = -0.7
$$

From this, we get that $m = -2.6$. However, such a tip would be the worst (it would differ from the correct tip by 2.3), so this option does not occur.

b) The average tip is 0.1. Then we have

$$
\frac{-1.2 + 1.7 + m}{3} = 0.1.
$$

From this, we get that $m = -0.2$.

Martin tipped $-0.2$.

[Finding the result without detailed analysis - 3 points, complete solution with indicated discussion - 6 points]