The missing number in the pattern $1,5,2,10,\dots ,15,4,20$ is:

The given sequence can be broken down into two alternating sub-series:First sub-series (at odd positions): $1,2,\dots ,4$ which increases by $+1$. Thus the missing term at the 5th spot should be $3$.Second sub-series (at even positions): $5,10,15,20$ which increases by $+5$.Let's align it sequentially:1st term: 12nd term: 53rd term: 24th term: 105th term (Missing): 36th term: 157th term: 48th term: 20Hence, the missing value is 3.