Maths problem: three nuts and six bolts have a combined mass of 72g. four nuts and 5 bolts have a combined mass of 66g. find the combined mass of one nut and one bolt

3n+6b=72, 

3n=72-6b

n=24-2b  and we are also told that:

4n+5b=66  substituting in n found above into this equation gives

4(24-2b)+5b=66

96-8b+5b=66

-3b=-30

b=10 grams, and since from earlier we saw that n=24-2b

n=24-2(10)

n=24-20

n=4 grams

So the combined mass of one bold and one nut is:

10+4=14 grams