Three pencils cost 75p altogether. how much will it cost to buy 6 pencils?

The cost for six pencils would be a total of 150p
The reason why is if three pencils cost 75p and you want to know what the cost of six, since three is half of six you would do this (if 3=75p then 3*2=75p*2 which is 6=150p). As shown, you multiply each side by 2 or double each side

Cost of 3 pencils = 75p
Cost of 1 pencils = 75/3
Cost of 6 pencils = [75/3]*6 = 75*2 = 150p.