"Zestig is een groot nummer."
Translation:Sixty is a big number.
Well... I don't know what your mother tongue is, so this may not be helpful at all, but...
'G' sounds like the 'ch' in 'loch' (as in Loch Ness), or like the Spanish 'j'. If you don't know how these sound, then think of an 'h', and then make the sound harder. (Someone here on the Duo discussions compared it to the sound cats make when they get annoyed/scared :D ).
After that sound comes the r... Which sounds like an r :D The r in Dutch is pronounced in different ways, depending on the speaker/regional accent. As far as I'm concerned, there's a rolling r (similar to the r in Spanish), which is mainly produced by Flemish speakers (in my experience); and the r of most nederlanders I've heard sound:
1) like the German/French r when initial in a syllable or before a vowel;
2) like the English r when in word final position or before a consonant.
Of course, I am not an expert, I'm just a learner here!