​​题目传送门​​ 代码：

#include<bits/stdc++.h>
using namespace std;

#define max(a,b) (a>b?a:b)
#define min(a,b) (a<b?a:b)
typedef long long LL;
const int mod=1e9+7;
int l[110],r[110];

LL mypow(LL a,LL b){
  
  LL sum=1;
  while(b){
    
    if(b&1) sum=sum*a%mod;
    a=a*a%mod;
    b>>=1;
  }
  return sum;
}

int main(){
  
  int T;
  scanf("%d",&T);
  while(T--){
    
    int n;
    scanf("%d",&n);
    LL sum=1;
    int Max=0,Min=0;
    for(int i=1;i<=n;i++) scanf("%d%d",&l[i],&r[i]),sum=sum*(r[i]-l[i]+1)%mod,Min=max(Min,l[i]),Max=max(Max,r[i]);
    LL fm=mypow(sum,mod-2);
    LL key=0;
    for(int i=Min;i<=Max;i++){
      
      LL tmp=1;
      for(int j=1;j<=n;j++){
        
        int L=l[j],R=min(i,r[j]);
        tmp=tmp*(2*i-L-R+2)*(R-L+1)/2%mod;
      }
      key=(key+tmp)%mod;
      tmp=1;
      for(int j=1;j<=n;j++){
        
        int L=l[j],R=min(i-1,r[j]);
        if(L>R){
          
           tmp=0;
           break;
          }
          tmp=tmp*(2*i-L-R+2)*(R-L+1)/2%mod;
      }
      key=(key+mod-tmp)%mod;
    }
    printf("%lld\n",key*fm%mod);
  }
}