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?

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**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