`3×(x/4+x/28+x/70+x/130)=60/13`
`3×x(1/4+1/28+1/70+1/130)=60/13`
`3×x(\frac{1}{1×4} +\frac{1}{4×7} +\frac{1}{7×10} +\frac{1}{10×13})=60/13`
`3×x×1/3×(\frac{3}{1×4} +\frac{3}{4×7} +\frac{3}{7×10} +\frac{3}{10×13})=\frac{60}{13}`
`x×(1-1/4+1/4-1/7+1/7-1/10+1/10-1/13)=\frac{60}{13}`
`x×(1-1/13)=\frac{60}{13}`
`x×(13/13-1/13)=\frac{60}{13}`
`x×12/13=\frac{60}{13}`
`x=\frac{60}{13}:12/13=60/13× 13/12`
`x=5.`
