Question

student pays 120 and 210 in 5th and 6th week How many weeks are required to accumulate 1680

Answer 1

tn=a+(n-1)d

t5=a+(5-1)d

120=a+4d.

a+4d=120.......eq1

also,

t6=a+(6-1)d

210=a+5d
a+5d=210......eq2

sub eq1-2
a+4d=120
-a+5d=210
________
-d=-90

d=90
put d=90 in eq 1
a+4×90=120
a+360=120
a=120-360
a=-240
now,
tn=1680
tn=a+(n-1)d
1680=-240+(n-1)90
1680=-240+90n-90
90n-330=1680
90n=1680+330
90n=2010
n=2010/90
n=201/9
n=67/3
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