Question 794721
t= tricycle
b-bicycle
d= tandem
u- unicycle

There are 57 helmets available (one for each rider).
t+b+2d+u=57
There are 115 wheels total.
3t+2b+2d+u=115


There are 32 regular bikes.
b=32


There are the same same number of tandem bikes as there are unicycles.
d=u

t+b+2d+u=57

3t+2b+2d+u=115

b=32

d=u

Substitute in the first equation

t+32+2d+d=57
t+3d=25>...............................(1)

substitute in second equation
3t+64+2d+d=115

3t+3d=115-64
3t+3d=51
t+d=17..................................(2)

solve 1 & 2 we get
 d= 4 Tandem bikes
t+d=17
t=13.... Tricycles

u = d 
so u =4