Answer: The value of a must be '5'.
Explanation:
Since we have given that
P(X≤a)=0.6
That means the value of area under the curve 'a' is 0.6.
So, We start adding the values for a to approach 0.6.
i.e.
P=0.05 upto a = 1
P= 0.1 upto a = 2
P = 0.15 upto a= 3
P = 0.2 upto a = 4
P= 0.2 upto a = 5
So, we get that

So, At a = 5, we get
P(X≤a)=0.6
Hence, the value of a must be '5'.