Question

plz solve :
t5/8-120 = t3/8
     OR

t3/8+120 = t5/8     

Answer 1

t3/8+120 = t5/8

5T/8 - 3T/8 = 120

2T/8 = 120
T/4 = 120
T=480

Answer 2

If the given problem corresponds to Arithmetic progression
then
tn = a + ( n - 1 ) d
where a = first term
         d = common difference  
t5 = a + 4d
t3 = a + 2d


now t5/8 -120 = t3/8

$\frac{a+4d}{8} - 120 = \frac{a+2d}{8}$

multiply the equation by 8 on both sides
then

a+4d - 960 = a+ 2d

a+4d -960 -a -2d =0
2d =960
d= 960 /2
 =480


we know the series in A.p are a, a+d, a+2d, a+3d,...etc

by hit and trial method let a=40 then the sequence becomes

40,520,1000,1480,1960,2440,(a+6d),....,a+(n-1)d.