An expression containing square roots of a whole number and two decimal numbers: $\sqrt{400} + \sqrt{0.09} - \sqrt{1.44}.$

Calculate the roots individually ($20, 0.3, 1.2$) and perform addition/subtraction. Notice the unit digit: $20.0 + 0.3 - 1.2$ ends in $0 + 3 - 2 = 1$, helping you quickly identify 19.1.

Misplacing the decimal point when calculating $\sqrt{0.09}$ (some students mistake it for $0.9$ instead of $0.3$) or $\sqrt{1.44}$ ($1.2$ vs $0.12$).

Mastering the square roots of perfect squares up to 30 and their decimal counterparts is essential for speed in Data Interpretation and Simplification sections of competitive exams.