When you say something like "three fifteents", you means that you have to consider the number 15 three times, so you're implicitly talking about multiplications.
So, "3 fifteens" means [tex] 3\cdot 15 [/tex] and in the same fashion "4 twos" means [tex] 4\cdot 2 [/tex]
Finally, "and" implies a sum, so the expression would be
[tex] 3\cdot 15+4\cdot 2 [/tex]
