Step-by-step explanation:
that is so simple, it is already difficult to make it formal, because there are only so few things to deal with. you really need help with this ?
you are not familiar with the situation "hey, this costs $10, we are 5 people, if we buy and then use it together, each one has to pay $2" ?
and this is just the other way around : if 5 people paid $2, how much did they pay together ? $10.
let's make it super formal :
price p = Allen contribution a + brother1 contribution b + brother2 contribution c
p = a + b + c
a = b = c = $32.30
p = $32.30 + $32.30 + $32.30 = 3×$32.30 = $96.90
