Question

A dozen of pens lost is 40 naira 80 kobo, find the cost of 10pens

Answer 1

This is clearly your homework, but either there is missing data or you have a golden opportunity to confuse your teacher.

let

a = price of a pen

b = price of a pencil

Your premise writes as 3*a + 5*b = 14 and it gives a = (14 - 5*b)/3

different values of b, give you values of a, for instance b=2 gives a=2/3=0,66666

or b=1 gives a=3 .

There is a condition missing. Probably that condition is that a and b be integers, in such case, the second solution would fit you as 10*1+12*5=70.

In that case this is called a Diophantine equation, concretely a lineal one, and a very particular case, called a Bézout identity and it is guaranteed to have a solution because the greatest common divisor of the coefficients (3 and 5) is 1 and also divides the independent term ( 14 ).