Although 2-pentanone is an isomer of 3-pentanone, the H nmr spectra are quite different.
2-Pentanone has four types of H : two different CH₂ units and two different the CH₃ units.
These are seen as four sets of peaks.
The CH₂ unit attached to the C=O is more deshielded (2.4 ppm) than the other CH₂ unit (1.6 ppm).
Likewise, the CH₃ unit attached to the C=O is more deshielded (2.13 ppm) than the other CH₃ unit (0.93 ppm).
The CH₃ unit attached to the C=O has no H neighbours, so we see just one line, a singlet.
The other CH₃ unit has 2 H neighbours, so the n+1 rule means we see the 3 lines of a triplet.
The CH₂ unit attached to the C=O has 2 H neighbours, so the n+1 rule means that we see the 3 lines of a triplet.
The other CH₂ unit has 2 + 3 = 5 H neighbours. Technically there are two different types of neighbours, but they are similar enough (i.e. both are sp3 centers)  that we can approximate and treat them together. So with the n+1 rule we see the 6 lines of a sextet.
Integrals have a 2:3:2:3 ratio