Answer
See below.
Work Step by Step
We know that $probability=\frac{\text{number of favorable outcomes}}{\text{number of all outcomes}}$
a) The number of all outcomes is $_{10}C_4=210$, out of these $_6C_4=15$ correspond to the desirable selection.
Hence here the probability is $\frac{15}{210}=\frac{1}{14}$
b) The number of all outcomes is $_{10}C_4=210$, out of these $_6C_2\cdot_4C_2=90$ correspond to the desirable selection.
Hence here the probability is $\frac{90}{210}=\frac{3}{7}$
