How many envelopes did they use ?

An office used three types of envelopes: small, medium and large.
They used 160 small envelopes.
The no. of medium envelopes used was 5 times the no. of small envelopes plus 5/20 of the total no. of the envelopes.
The no. of large envelopes was equal to the no. of small envelopes plus 5/20 of the no. of medium envelopes.
Altogether, How many envelopes did they use?

Click here for Answer

Answer is 1920

Solution :
Small- S, Medium- M, Large- L, Total- T

S= 160
M= 5*S+T*5/20= 800 + T/4
L= S+ M*5/20 = 160 +(800+T/4) *5/20= 360 + T/16
T=S+M+L
T= 160+ 800+ T/4 + 360 + T/16 => T=1920