Using the property of binomial coefficients, we know the recursive addition property: ${}^{n+1}C_r = {}^nC_r + {}^nC_{r-1}$. Applying this property iteratively, we can combine the terms as follows: \[ {}^{49}C_3 + {}^{48}C_3 + {}^{47}C_3 + {}^{46}C_3 + {}^{45}C_3 + {}^{45}C_4 = {}^{50}C_4 \] This sum represents the binomial coefficient ${}^{50}C_4$, as derived from the addition property of binomial coefficients.
Number of 4-digit numbers (the repetition of digits is allowed) which are made using the digits 1,2 , 3 and 5 , and are divisible by 15 , is equal to _____