SOL 
Let the number of tickets Mr Tan has be X . Let the number of companies be Y According to first condition , X + 20 = 4Y (he sends 4 tickets each to Y companies . he will need 20 more )
According to second condition , X50 =3Y ( he sends 3 tickets each to Y companies , 50 tickets remains with him )
NOW lets rearrange these 2 equations and solve them simultaneously
We get X4Y = 20 (1)
X3Y = 50 (2)
Substacting equation 2 from equation 1 we get ,
X4Y=20

X 3Y = 50
 + 
4Y + 3Y = 20  50
Y = 70
Y =70 ( 3 )
Substituting the value of Y =70 in equation 1 we get the value of X .
X 4 * 70 = 20
X  280 = 20
X = 20 + 280
X = 260
Answer  Mr Tan has 260 tickets and number of companies equals to 70 .
Let the number of tickets he has be X
Let the number of companies be Y
so 4*Y=X+20
3*Y=X50
Thus we have two equations in two variables,
4*Y=X+20
3*Y=X50
We want to find how many tickets does he have that means we want to find X,
So we have to eliminate Y from two equations.
From first equation,
Y=(X+20)/4
Substituting value of Yin second equation,
3*Y=X50
3*(X+20)/4=X50
3*(X+20)=4*(X50)
3X+60=4X200
4x3x =200+60
X=260
So he has 260 tickets in his hand.
Number of companies is Y=(X+20)/4=260+20/4 = 280/4 = 70
when he give 4 tickets to 70 companies, he needs 280 tickets. but he has only 260 and so he needs 20 more.
when he gives 3 tickets to 70 companies, he only want 210 tickets and so he has 50 tickets more in his hand.
So the total number of tickets in his hand is 260 tickets.