Question 507064
Let x = number of old pennies, y = number of new pennies


Since you "have 15 pennies total", this means that {{{x+y=15}}} and solving for y gives you {{{y=15-x}}}



Because you know that the mass of the old pennies are 3.10g and new pennies are 2.50gm, this means that {{{3.1x+2.5y=T}}} where T is the total weight of all the coins combined.



Now plug in {{{y=15-x}}} to get {{{3.1x+2.5(15-x)=T}}}



Now if you knew the total weight T, then you could solve for x to find the number of old pennies in the jar.