Let the numbers be A,B,C,D,E,F,G. and their average is 20 so the total sum of these numbers would be 20*7=140 (averaging them would mean 140/7=20)
Now for the first four numbers A,B,C,D the average is 19 so their total sum would be 19*4=76
For the last four numbers D,E,F,G the average is 24 so their total sum would be 24*4=96
Now If we sum these two quantities we would get the sum of (A,B,C,D)+(D,E,F,G)=76+96=172
That is A+B+C+2D+E+F+G=172
But we know from the first line that the sum of all numbers is A+B+C+D+E+F+G=140
Subtracting this from the previous equation would mean (A+B+C+2D+E+F+G)-(A+B+C+D+E+F+G)=172-140 = D=32