ParametricNDSolveValue with coupled DEs and manipulation

Question

How do I separate the solutions to these coupled DEs in the plot?

I want to plot the solutions to these equations at time t=7 as a function of parameter a while manipulating parameter b.

Code:

Manipulate[    
 sol = ParametricNDSolveValue[
   {s'[t] == -a*b*d[t], d'[t] == +b*s[t], s[0] == 100, d[0] == 0},
   {s[7], d[7]},
   {t, 0, 20},
   {a}
   ];

 Show[{
   Plot[sol[a], {a, 0.1, 10}, ImageSize -> 400, 
    PlotStyle -> {Black, Blue}, PlotLabel -> populations]
   }],

 Text["Test-----"],
 {{b, 1, "b"}, 0.1, 10, Appearance -> "Open"}
 ]

I tried

Plot[Evaluate@Table[(s[t],d[t])/.sol[a]],{a,0.1,10}]

and

Plot[Evaluate@Table[(s[a][t],d[a][t])/.sol],{a,0.1,10}]

and various other lines but Mathematica is giving me errors like "is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing"

Answer 1

Two changes in your code: (1) made sol a two-parameter function and evaluate it outside Manipulate, (2) used {sol[a,b][[1]],sol[a,b][[2]]} as the first argument in Plot:

sol = ParametricNDSolveValue[{s'[t] == -a*c*d[t], d'[t] == c*s[t], 
    s[0] == 100, d[0] == 0}, {s[7], d[7]}, {t, 0, 20}, {a, c}];

Manipulate[Plot[{sol[a, b][[1]], sol[a, b][[2]]}, {a, 1/10, 10}, 
  PerformanceGoal -> "Speed", ImageSize -> 400, 
  PlotStyle -> {Black, Blue}], 
 {{b, 1, "b"}, .1, 10, Appearance -> "Open"}]

Note: If we use sol[a,b] or Evaluate@sol[a,b] instead of {sol[a, b][[1]], sol[a, b][[2]]} we get two curves with the same color.